[web] Honor requested upscaling in instantiateImageCodecWithSize and ImageDescriptor.instantiateCodec - #195836

On the web, instantiateImageCodecWithSize and ImageDescriptor.instantiateCodec (encoded path) forwarded to the codec with a hardcoded allowUpscaling: false. Any requested target size larger than the intrinsic size was silently clamped back down, so ResizeImage(allowUpscaling: true) could never upscale on web.

This PR removes the hardcoded flag so both entry points decode to the size the caller asked for, matching native dart:ui. The upscaling policy already lives in the framework: ResizeImage's getTargetSize clamps to the intrinsic size when its own allowUpscaling is false, so that case behaves exactly as before.

Changes:

• engine/src/flutter/lib/web_ui/lib/painting.dart: drop allowUpscaling: false in instantiateImageCodecWithSize and in the encoded branch of ImageDescriptor.instantiateCodec. Every callee defaults to allowUpscaling: true, so this enables upscaling uniformly across the ImageDecoder, createImageBitmap, and Skia fallback paths.
• engine/src/flutter/lib/web_ui/test/ui/image_test.dart: new tests that decode a 10x10 image to larger sizes through both entry points, including non-square, mixed up/down, and width-only (aspect-preserving) targets. The existing Image.onCreate/onDispose hook resets now use addTearDown, so a failing assertion can't leak hooks into later tests.
• packages/flutter/test/painting/image_provider_resize_image_test.dart: remove skip: isBrowser from group('resizing'). Its 43 tests now run on web and pass on CanvasKit and Skwasm.

Fixes #195193

Where to focus

• Key decision: honor the caller's size instead of adding a web-only knob. The public dart:ui signatures are unchanged. The web engine now trusts the target size it is given, as native does, rather than applying a second, stricter upscaling policy underneath the framework's.
• Non-obvious: the argument is removed, not set to true. true is already the default at every hop (engineInstantiateImageCodec → browser/static/Skia codecs → scaleImageIfNeeded), and writing it explicitly trips avoid_redundant_argument_values.
• Non-obvious: the old clamp wasn't applied evenly. The Skwasm Skia fallback already resized natively and ignored the flag. After this change, every decode path behaves the same.
• Primary review file: engine/src/flutter/lib/web_ui/lib/painting.dart (two call sites). Then engine/src/flutter/lib/web_ui/test/ui/image_test.dart.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant in-code documentation (doc comments with ///).
• If this PR introduces a new feature or capability, I created and linked a website documentation issue or PR in flutter/website (or verified none is needed).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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