feat(web): emit precache_manifest.json when --web-content-hash is enabled - #192953

…hide --web-content-hash (flutter#193040)

## Description

Completes the `--web-content-hash` architecture by adding runtime
raw-key asset resolution in `web_ui`, achieving 100% content-hashing
coverage across all files in `build/web/assets/` (eliminating manifest
and shader exclusions), preserving `AssetManifest.listAssets()` and
`AssetMetadata.main`, validating custom `web/flutter_bootstrap.js` /
`web/index.html` templates, and un-hiding `--web-content-hash`
(`--no-web-content-hash`) in `flutter build web --help`.

### Why this is needed

Phase 2 (flutter#191919) and Phase 3
(flutter#192953) renamed physical asset
files on disk (`<name>.<hash>.<ext>`), rewrote target values inside
`AssetManifest.bin`, `AssetManifest.bin.json`, `AssetManifest.json`, and
`FontManifest.json`, and emitted `precache_manifest.json`. Two critical
gaps remained before exposing `--web-content-hash` to users:

1. **Cold-cache 404s on raw-key asset requests**:
- While `Image.asset` resolves variant paths via `AssetManifest` before
calling `rootBundle.load(variant.key)`, direct callers of
`rootBundle.load(key)` / `rootBundle.loadString(key)`
(`PlatformAssetBundle`) and third-party web plugins (`vector_graphics` /
`flutter_svg` `AssetBytesLoader`, `lottie`, `rive`, `video_player_web`,
`audioplayers_web`) pass the raw declared key
(`assets/data/config.json`) directly to
`ui_web.AssetManager.getAssetUrl(String asset)`.
- Without runtime key translation in
`ui_web.AssetManager.getAssetUrl(asset)`, raw-key asset fetches
requested un-hashed paths (`assets/assets/data/config.json`) and 404'd
on cold caches.
2. **Un-hashed manifest and shader files under `build/web/assets/`**:
- `AssetManifest.*`, `FontManifest.json`, `NOTICES`, `NOTICES.Z`, and
`shaders/**` were previously excluded from hashing, which meant a single
`"source": "assets/**" -> Cache-Control: public, max-age=31536000,
immutable` CDN rule would freeze `AssetManifest.bin.json` and
`FontManifest.json` across deployments (`v1 -> v2`).

### What this PR changes

1. **100% `build/web/assets/` Immutability & Clean
`AssetManifest.listAssets()`
(`packages/flutter_tools/lib/src/web/content_hash.dart`)**:
- Removes `_excludedFilenames` and `_excludedDirPrefixes` so every file
under `build/web/assets/` (including `shaders/**`, `NOTICES`,
`NOTICES.Z`, `FontManifest.json`, `AssetManifest.json`,
`AssetManifest.bin`, and `AssetManifest.bin.json`) is content-hashed on
disk.
- Keeps `AssetManifest.bin` and `AssetManifest.json` strictly limited to
user/package manifest entries (storing `Uri.decodeFull` variant paths so
`Image.asset` on spaced filenames encodes cleanly to `%2520` rather than
triple-encoding to `%252520`), and passes non-manifest SDK files
(`NOTICES`, `shaders/ink_sparkle.frag`, `shaders/stretch_effect.frag`,
`AssetManifest.bin`, `AssetManifest.json`) via
`_flutter.buildConfig.extraAssets` (~200 bytes) alongside
`"assetManifest"` and `"fontManifest"`.
- Sorts discovered asset paths deterministically before manifest
assembly so incremental and clean builds emit identical manifest hashes.
- Fails fast (`throwToolExit`) if custom `web/flutter_bootstrap.js` or
`web/index.html` templates omit `{{flutter_build_config}}` or contain
malformed `_flutter.buildConfig` assignments.
2. **Preserve `AssetMetadata.main`
(`packages/flutter/lib/src/services/asset_manifest.dart`)**:
- Updates `_AssetManifestBin` (`asset_manifest.dart`) so
`AssetMetadata.main` remains `true` for the primary 1.0x asset variant
when content hashes are embedded in variant filenames.
3. **Incremental Build `.filecache` Tracking
(`packages/flutter_tools/lib/src/build_system/targets/web.dart`)**:
- Declares `{OUTPUT_DIR}/*/index.html` and
`{OUTPUT_DIR}/flutter_bootstrap.js` in `WebReleaseBundle.inputs` and
`WebReleaseBundle.outputs` when `webContentHash` is enabled so
`.filecache` records post-injection file hashes.
4. **Synchronous Engine-Private Runtime Key Translation (`web_ui`)**:
- Extends `FlutterJS` with `JSBuildConfig` (`assetManifest`,
`fontManifest`, `extraAssets`) in `js_loader.dart`.
- Updates `fetchFontManifest` in `fonts.dart` to fetch
`flutter?.buildConfig?.fontManifest?.toDart ?? 'FontManifest.json'`.
- Adds `_loadContentHashedAssetManifest(ui_web.assetManager)` inside
`initializeEngineServices()` `Future.wait` in `initialization.dart` to
register `assetManifest`, `fontManifest`, `extraAssets`, and decoded
`AssetManifest.bin.<hash>.json` mappings in engine-private
`setContentHashedAssetMap` / `resolveContentHashedAsset` (keeping the
public `dart:ui_web` `AssetManager` API unchanged).
- Resolves logical keys and variant paths synchronously inside
`ui_web.AssetManager.getAssetUrl(String asset)`.
5. **Un-hide `--web-content-hash`
(`packages/flutter_tools/lib/src/web/web_options.dart` &
`build_web.dart`)**:
- Removes `hide: true` from `WebOptions.webContentHash` and notes that
local `canvaskit/**` (`--no-web-resources-cdn`) remains un-hashed
(`urlHashed: false` in `precache_manifest.json`).

## Related Issues

- Fixes flutter#191915
- Fixes flutter#193031
- Part of flutter#149031

## Tests

-
`packages/flutter_tools/test/general.shard/build_system/targets/web_test.dart`
-
`packages/flutter_tools/test/commands.shard/hermetic/build_web_test.dart`
& `args_test.dart`
- `packages/flutter/test/services/asset_manifest_test.dart`
- `engine/src/flutter/lib/web_ui/test/engine/assets_test.dart`

## Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.