fix: Check disabled state before AI Config mode mismatch

[jsonbailey]

Bug

A disabled AI Config variation is served as {"_ldMeta": {"enabled": false}} with no mode field. In ConfigFactory, each of the three builders (BuildCompletionConfig, BuildAgentConfig, BuildJudgeConfig) ran the mode-mismatch check before looking at the enabled flag. Because ParseMeta defaults a missing mode to "completion", an agent or judge caller whose served variation was the disabled/off variation saw a spurious warning:

AI Config mode mismatch for sample-agent: expected agent, got completion. Returning caller's default.

The config wasn't a mode mismatch at all — it was simply disabled.

Fix

In all three builders, the disabled check now runs before the mode comparison. When the served variation is disabled (enabled == false):

• No mode comparison runs and no "mode mismatch" warning is emitted.
• The builder returns a disabled config of the requested type (LdAiCompletionConfig / LdAiAgentConfig / LdAiJudgeConfig) with enabled: false, carrying through the served variationKey and version.

A minimal disabled config of the requested type is built rather than parsing the served body, since a disabled variation carries no model/messages/instructions. Null model/provider is safe — the base LdAiConfig constructor already coalesces them to empty defaults, matching how the SDK treats disabled configs elsewhere.

The genuine mode-mismatch behavior for enabled configs whose mode truly differs is unchanged: it still warns and returns the caller's default. No public API changes.

Tests

Added two regression tests in LdAiClientAgentJudgeTest: calling the agent and judge builders with a served {"_ldMeta": {"enabled": false}} (no mode) returns a disabled config of the requested type and emits no mode-mismatch warning. Existing mismatch tests (enabled config, wrong mode) continue to assert the warn-and-default path.

Related

Companion changes are going up on branch jb/disabled-config-mode-check in:

• sdk-specs (spec change)
• js-core (companion SDK fix)