[web] Support runtime content-hashed asset resolution in web_ui and un-hide --web-content-hash #193031

Summary

Part of umbrella issue #149031.

With physical static assets now content-hashed on disk during flutter build web --web-content-hash (#191915 / #191919), calls to load assets by raw unhashed key (rootBundle.load, rootBundle.loadString, vector_graphics AssetBytesLoader, video_player_web, and DOM media plugins) encounter 404 errors on fresh browser caches because runtime code requests unhashed logical paths directly over the 'flutter/assets' platform channel or via ui_web.assetManager.getAssetUrl(key).

Additionally, serving asset manifests (AssetManifest.bin.json, FontManifest.json) with fixed unhashed filenames prevents web hosting configurations from caching assets/** with Cache-Control: immutable without complex no-cache carve-outs for manifests, shaders, and notices.

Scope & Requirements

1. Web Engine Runtime Asset Resolution (ui_web.AssetManager):
   • In engine/src/flutter/lib/web_ui/, teach ui_web.AssetManager to resolve raw logical keys (assets/data/config.json) to content-hashed paths synchronously via AssetManager.getAssetUrl(asset).
   • In initialization.dart, ingest the key-to-hash mapping during initializeEngineServices() (via _flutter.buildConfig or AssetManifest.bin.json) before user code executes.
   • Ensure resolution is idempotent for already-hashed DPR variants produced by AssetImage.
2. 100% Asset Directory Immutability:
   • Remove exclusions in packages/flutter_tools/lib/src/web/content_hash.dart for shaders/, NOTICES, and NOTICES.Z, hashing them on disk along with AssetManifest.bin.json and FontManifest.json.
   • Pass manifest hashes via _flutter.buildConfig in flutter_bootstrap.js so all files under build/web/assets/ can be served with Cache-Control: max-age=31536000, immutable.
3. Un-hide --web-content-hash:
   • Remove hide: true from WebOptions.webContentHash in packages/flutter_tools/lib/src/web/web_options.dart once runtime resolution and manifest immutability pass browser integration verification.