Add content hashes to asset URLs

[harsh21234i]

Closes #6550.

Adds content-hash cache busting to rx.asset URLs by appending an 8-character SHA-256 query parameter based on the
asset file contents.

Example:
rx.asset("logo.svg") now returns /logo.svg?v=<hash>.

The hash changes only when the file content changes, allowing browsers to cache assets safely across deploys.

Implementation notes:

• Applies to both local and shared assets.
• Keeps AssetPathStr.importable_path unversioned so existing build-time import behavior is preserved.
• Preserves backend-only behavior for missing local assets.
• Adds regression coverage for hash changes and pickle/copy round trips.

[greptile-apps]

Greptile Summary

This PR adds content-hash cache busting to asset URLs. The main changes are:

• Appends an eight-character SHA-256 value to local and shared asset URLs.
• Preserves unversioned paths for build-time imports and serialization.
• Reads files in chunks and retries when content changes during hashing.
• Documents the new URL format and adds unit coverage.

Confidence Score: 5/5

No new blocking issue was identified in this follow-up.

• The updated hashing code retries when consecutive reads differ.
• Tests cover content changes, replacement, in-place rewrites, fallback behavior, and serialization.
• No separate production failure requiring an additional comment was found.

Important Files Changed

Filename	Overview
reflex/assets.py	Adds versioned asset paths, chunked hashing, stability retries, and serialization support.
tests/units/assets/test_assets.py	Adds coverage for hash changes, retry behavior, fallback behavior, and serialization.
docs/assets/overview.md	Documents content-hash query parameters for asset URLs.
news/6550.feature.md	Adds a release note for asset cache busting.

_{Reviews (6): Last reviewed commit: "Verify asset hashes with consecutive con..." | Re-trigger Greptile}

[codspeed-hq]

Merging this PR will not alter performance

✅ 26 untouched benchmarks
⏩ 8 skipped benchmarks¹

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_{Comparing harsh21234i:fix/asset-content-hash-6550 (4f2e7b9) with main (32cc257)}

Footnotes

1. 8 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩