[web] Report skwasm WebGL context creation failure instead of hanging - #193293

When skwasm fails to create a WebGL2 context on the worker (e.g. Chrome launched with --use-gl=angle --use-angle=gl --enable-gpu-rasterization --ignore-gpu-blocklist in some environments), Surface::ReceiveCanvasOnWorker printed Failed to create context! and returned without calling skwasm_reportInitialized. As a result:

1. SkwasmSurface._initializedCompleter never completed.
2. setSize / rasterizeToImageBitmaps awaited initialized forever, wedging the render loop (later scenes only overwrite renderQueue.next).
3. Platform views injected during the synchronous measure pass of the first frame appeared, but any platform view mounted later (e.g. the Google Identity Services button container in the issue's headless repro) was never attached to the DOM. No error surfaced to Dart.

Before #179473 this path crashed loudly (#173401); since then it hangs silently.

Fix

• C++/JS: skwasm_reportInitialized / surface_onInitialized gain a success flag. The failure path now reports initialization with success = false, and OnInitialized passes 0 (never a valid callback id, since ids start at ≥ 1 and the context-lost id is always ≥ 2) instead of the context-lost callback id. _skwasm_getGlContextForCanvas also returns 0 early when GL.createContext fails so it does not store key 0 in handleToCanvasMap or attach a webglcontextlost listener.
• Dart: SkwasmSurface._initialize treats a 0 id as failure, sets _initializationFailed = true, and completes initialized with a StateError describing the WebGL2 failure instead of hanging. Subsequent rasterizeToImageBitmaps and setSize calls return early once _initializationFailed is set so later frames fail quietly rather than rethrowing on every frame, and onContextLost catches errors from recreateContextForCanvas so post-crash context recreation failures do not escape as duplicate unhandled async errors.
• Test: new tests in test/ui/surface_context_lost_test.dart covering both initial WebGL context creation failure and context recreation failure after context loss.

Verification

• Reproduced the hang with a headless Chrome harness using the reporter's GPU flags (skwasm, master): Failed to create context! in console, later-mounted flt-platform-view never attached. Default flags / --disable-gpu unaffected.
• felt test --gcs-prod --suite chrome-force-st-dart2wasm-skwasm-ui test/ui/surface_context_lost_test.dart:
  • against stock prebuilt skwasm: new test fails with TimeoutException (the hang); existing 4 pass.
  • with the prebuilt skwasm.js glue patched to emulate this C++ change: 5/5 pass.
• App-level with patched platform dill + emulated glue: clear Bad state: Skwasm failed to create a WebGL2 context… error instead of silent hang; no regression with default flags.

Related to #193223 (partial: headless/GPU-flags variant only; intentionally not Fixes, so merging won't close the issue)

Pre-launch Checklist

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