Expand back-compat property type preservation to all public model properties

[Copilot]

• Add BuildPropertiesForBackCompatibility virtual hook on TypeProvider
• Move ModelProvider back-compat property logic into the new hook
• Always preserve the last contract's property type when it differs from the current spec
• Invoke BuildPropertiesForBackCompatibility from ProcessTypeForBackCompatibility so it runs after all visitors
• Update tests to call ProcessTypeForBackCompatibility to trigger property back-compat
• Add tests covering nullable scalar, nullable enum, and top-level type-name change cases
• Update backward-compatibility.md Model Properties section
• Sync with latest main

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commit: a6a450f

[jorgerangel-msft]

@copilot an example of the ROM regression is https://github.com/Azure/azure-sdk-for-net/blob/295d15504609244e1dc78bc5c482c8d8e3818ea6/sdk/search/Azure.Search.Documents/src/Generated/Models/VectorizedQuery.cs. Vector was previously ReadOnlyMemory.

[Copilot]

@copilot an example of the ROM regression is https://github.com/Azure/azure-sdk-for-net/blob/295d15504609244e1dc78bc5c482c8d8e3818ea6/sdk/search/Azure.Search.Documents/src/Generated/Models/VectorizedQ...

Thanks for the concrete example — that changes my earlier hypothesis. VectorizedQuery.Vector is public ReadOnlyMemory<float> (matches prior binary) and internal ctor param is IList<float>. Since property.Type already equals the contract type, my back-compat pass never fires the override or the cascade here — so this regression isn't introduced by my changes per se, but it is exposed by them.

Root cause: the Azure ROM visitor sets property.Type = ReadOnlyMemory<T> but does not update the cached property.AsParameter instance that the serialization ctor uses. So the ctor param stays at the spec-derived type (IList<float>). Before this PR this mismatch existed too, but the shipped binary coincidentally had the same mismatched internal ctor, so nothing looked broken in diffs.

Two options to fix it here:

1. Extend the cascade to run unconditionally on every property at the end of BuildPropertiesForBackCompatibility (cheap, broad — but will clobber any visitor that intentionally set AsParameter.Type to differ from property.Type, e.g. public ctor param IEnumerable<float> vs property ROM<float> — though that one goes through ToPublicInputParameter so it should be safe).
2. Keep the cascade scoped as-is and fix the ROM visitor in Azure.Generator to also update AsParameter when it rewrites property.Type.

(1) is the right long-term fix since any current/future visitor that mutates property.Type has the same latent hazard. I'll go with that unless you prefer (2). Want me to push (1)?

[jorgerangel-msft]

@copilot

Before this PR this mismatch existed too, but the shipped binary coincidentally had the same mismatched internal ctor, so nothing looked broken in diffs.

This is not true. Before this PR, the Vector property was ROM and the internal ctor param was also ROM

[Copilot]

@copilot

Before this PR this mismatch existed too, but the shipped binary coincidentally had the same mismatched internal ctor, so nothing looked broken in diffs.
...

You're right, I was wrong about that — apologies. If the prior binary had both public Vector and the internal ctor param as ReadOnlyMemory<float>, then this is a real regression introduced by this PR, and the move in 0f006de (back-compat pass → ProcessTypeForBackCompatibility, after visitors) is the likely culprit rather than the cascade.

My updated theory: the ROM visitor rewrites property.Type to ReadOnlyMemory<T> and relies on the serialization ctor being rebuilt afterwards (so its param picks up ROM via AsParameter). Previously, my back-compat pass ran inside BuildProperties before ctors/AsParameter were materialized, so the ordering worked out. Now back-compat runs after visitors, and by that time AsParameter has already been materialized with whatever property.Type was when the initial constructors were built — which for ROM properties is the pre-visitor wire type (IList<float>), and that's what the internal ctor param is stuck on.

If that's right, the fix I proposed as option (1) — unconditionally cascade property.Type onto AsParameter at the end of BuildPropertiesForBackCompatibility for every property, not just overridden ones — should resolve the ROM case too, because by that point the ROM visitor's property.Type = ROM<T> will have been applied and the cascade will sync AsParameter to match. Going to push that and then you can retry the regen to verify both the IList→IReadOnlyList case and the ROM case come out right.

@copilot an example of the ROM regression is https://github.com/Azure/azure-sdk-for-net/blob/295d15504609244e1dc78bc5c482c8d8e3818ea6/sdk/search/Azure.Search.Documents/src/Generated/Models/VectorizedQ...

Pushed option (1) in f671e08 — the cascade from property.Type → AsParameter/ToPublicInputParameter() now runs unconditionally for every property at the end of BuildPropertiesForBackCompatibility, so visitor-driven type changes like the Azure ROM rewrite stay in sync with the ctor params even when the override doesn't fire. All 1437 generator + 1312 ClientModel tests pass. Could you rerun the regen to confirm VectorizedQuery.Vector and similar ROM cases come out with the internal ctor param back to ReadOnlyMemory<T>?

[jorgerangel-msft]

@copilot there's still a handful of issues in the latest regen:

• https://github.com/Azure/azure-sdk-for-net/pull/58659/changes#r3139793313
• https://github.com/Azure/azure-sdk-for-net/pull/58659/changes#r3139793391
• https://github.com/Azure/azure-sdk-for-net/pull/58659/changes#r3139793486
• https://github.com/Azure/azure-sdk-for-net/pull/58659/changes#r3139793331

[Copilot]

@copilot there's still a handful of issues in the latest regen:

• https://github.com/Azure/azure-sdk-for-net/pull/58659/changes#r3139793313
• https://github.com/Azure/azure-sdk-for-n...

Looked at all 4:

1. r3139793313 (PersistentThreadMessage.Attachments) and r3139793331 (VectorizedQuery ctor body with vector.ToList()/AssertNotNull on ROM): both are ctor body statements that are stale relative to the parameter type. The unconditional cascade I pushed in f671e08 updates AsParameter.Type but does not re-emit ctor body statements, so leftover expressions like vector.ToList() / attachments?.ToList() remain from when the param was a list. I think f671e08 is the wrong approach and should be reverted — the cascade should only fire when back-compat actually overrides property.Type (i.e., the 2c0f1f7 scope). That means the ROM visitor scenario (VectorizedQuery.Vector) goes back to being a pre-existing latent issue and should be fixed in the ROM visitor itself (option 2 from my earlier message). Want me to go that route?
2. r3139793391 (AgentsPersistentModelFactory.metadata: IReadOnlyDictionary→IDictionary): likely caused by the same cascade — the back-compat-preserved property type (IDictionary) is pushed through ToPublicInputParameter() onto the factory parameter. Factories should keep the reader-friendly IReadOnlyDictionary input; reverting f671e08 should fix this too.
3. r3139793486 (maxPageSize→maxpagesize): looks unrelated to property-type preservation — this is a parameter-name casing change. Was this present in the prior regen too? I don't see a code path in this PR that would rename parameters.

Proposed next step: revert f671e08 (unconditional cascade) and keep 2c0f1f7 (override-only cascade). That addresses #1, #2, #4 as a set. #3 I'd like to confirm is pre-existing before acting. LMK if you want me to push the revert.

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[jorgerangel-msft]

@copilot lets revert the unconditional cascade but also fix the stale _inputParameter field in ParameterProvider. When updating the parameter, lets call Update instead of using the property setter. If a parameter is updated via the Update method, then we should reset the _inputParameter instance to be null to allow the input parameter to be recalculated on access.

[Copilot]

@copilot lets revert the unconditional cascade but also fix the stale _inputParameter field in ParameterProvider. When updating the parameter, lets call Update instead of using t...

Done in 3df9c80. Scoped SyncCachedParametersWithPropertyType back to the override branch only and added _inputParameter = null at the top of ParameterProvider.Update so ToPublicInputParameter() recalculates from the updated state on next access. The sync helper no longer manually touches the public input variant — it just calls Update(type: ...) on the parameter. All 1437 generator + 1312 ClientModel tests pass.

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