Test/web camera release review feedback

[snodnipper]

Follow-up to juliansteenbakker#1779, addressing @navaronbracke's nine review comments on the tests, plus one gap found in the start path while doing so.

Review feedback

• The duplicated MediaStream helpers and start options move to test/web/utils/media_stream_test_utils.dart, shared by both web tests.
• getUserMedia and BarcodeDetector are stubbed through extension type bindings instead of eval. Restoring uses delete, which exposes the native MediaDevices.prototype.getUserMedia again, or leaves BarcodeDetector absent on browsers that never had it (so isSupported() feature detection stays honest). createJSInteropWrapper doesn't fit here: it returns a plain object, never a constructor, so it cannot stand in for BarcodeDetector, and navigator.mediaDevices is a getter-only accessor, so a wrapper can't be assigned there. I verified that new on a .toJS function raises the Dart error under both dart2js and dart2wasm.
• The dispose-during-start test now asserts directly that the disposed controller does not hold the camera session, through a @visibleForTesting platformSessionOwner getter next to the existing resetPlatformSessionOwner(). The behavioural check (the next controller's dispose reaches the platform) is kept. The unused stopCalls counter is gone.
• The teardown expectations have reasons, and the stop-twice test asserts that the second stop() completes.
• allLive / allEnded now fail on a stream with no tracks instead of passing vacuously.
• awaitCall() no longer busy-waits. If getUserMedia is never called, it fails after 5s with a TestFailure instead of hanging until the runner's 30s timeout.

Start-path gap

start() awaits the reader's library load (a real script load for zxing-wasm) before calling getUserMedia, with no teardown check in between. A scanner stopped in that window still prompted for permission and briefly lit the camera before the post-acquire check released it. start() now checks the teardown generation immediately before acquiring the camera. A new test fails without the fix (getUserMedia is called once) and passes with it.

Verified

• flutter test: 386 passed.
• test/web/polling_barcode_reader_stop_test.dart and test/web/mobile_scanner_web_start_teardown_test.dart pass in Chrome under dart2js and --wasm.
• Swapping develop's lib/ back in fails every new test.
• test/web/web_library_versions_test.dart fails under --platform chrome on develop too (it reads package.json from disk). That failure is unrelated.

Related: juliansteenbakker#1778

Checklist

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