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

---

_{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. ↩

[masenf]

seems like a useless extra initialization; just keep the one in the loop

[masenf]

Suggested change

	break
	return digest.hexdigest()[:8]
	return digest.hexdigest()[:8]
	console.warning(f"{path} was modified {_MAX_HASH_ATTEMPTS} times while calculating hash")
	return str(time.time())

we should consider doing something reasonable when we can't get a stable hash, like using the compile timestamp as the hash rather than always returning the last digest, which might cause the browser to load a previous cached version. at least if we take the timestamp, the browser will reload the asset once per compile, which is probably good enough for most uses.