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 query parameter to local and shared asset URLs.
• Reads asset files in chunks and retries when a file changes during hashing.
• Keeps build-time import paths free of the version query.
• Preserves versioned paths through pickle and copy operations.
• Documents and tests the new behavior.

Confidence Score: 4/5

The remaining cache race should be fixed before merging.

• Chunked hashing avoids loading entire assets into memory.
• Retries handle changes that occur while a file is being read.
• A change between hashing and the later frontend copy can still deploy new bytes under an old cache key.

reflex/assets.py

Important Files Changed

Filename	Overview
reflex/assets.py	Adds chunked content hashing, change detection, versioned asset paths, and serialization support; hashing can still diverge from the later build copy.
tests/units/assets/test_assets.py	Covers content changes, chunked reads, transient concurrent modification, import paths, and serialization round trips.
docs/assets/overview.md	Documents content-hash query parameters for asset URLs.
news/6550.feature.md	Adds a release note for content-hash cache busting.

_{Reviews (3): Last reviewed commit: "Merge branch 'main' into fix/asset-conte..." | 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 (9953842) with main (11f8a7a)}

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. ↩

[greptile-apps]

Hash Can Precede Copy — The stability check only covers the time spent reading source_file. The assets directory is copied into the frontend build later. If a generated asset changes after this hash is calculated but before that copy, the page keeps the old ?v= value while the public directory receives new bytes. Browsers may then reuse a cached response instead of fetching the deployed file. Hashing and copying need to use the same immutable snapshot.