[web] Keep ImageFilter.blur bounds in equality and description - #196013

On the web, ImageFilter.blur accepted the optional bounds argument and then dropped it. As a result, a bounded blur compared equal to the matching unbounded blur, and its description had no bounds: part. Native keeps bounds in ==, hashCode and debugShortDescription.

This PR forwards bounds from the web dart:ui ImageFilter.blur to EngineBlurImageFilter, which now stores it and includes it in ==, hashCode and debugShortDescription/toString(), in the same format as native _GaussianBlurImageFilter (blur(10.1, 10.1, clamp, bounds: Rect.fromLTRB(...))). Rendering does not change: every web backend still draws an unbounded blur, which is also what the native Skia backend does with bounds. With framework-visible behavior now matching native, 'ImageFilter resolves correctly' in image_filter_config_test.dart no longer needs skip: kIsWeb.

Part of #175899

Behavioral Changes

• Web ImageFilter.blur(..., bounds: r) equality:
  • Before: equal to the same blur without bounds (and to one with any other bounds).
  • After: equal only to a blur with the same sigmas, tile mode and an equal bounds, and hashCode follows the same rule. As on native, a layer or widget whose filter changes only in bounds now counts as changed.
• Web ImageFilter.blur(sigmaX: 10.1, sigmaY: 10.1, tileMode: TileMode.clamp, bounds: Rect.fromLTRB(0.1, 0.1, 100.2, 100.2)) description:
  • Before: debugShortDescription was blur(10.1, 10.1, clamp).
  • After: it is blur(10.1, 10.1, clamp, bounds: Rect.fromLTRB(0.1, 0.1, 100.2, 100.2)) (toString() gets the ImageFilter. prefix), matching native.
• Blurs without bounds behave exactly as before. Rendered output does not change on any web backend.

Where to focus

• Primary review file: engine/src/flutter/lib/web_ui/lib/src/engine/primitives/image_filter.dart. bounds is an optional, final field on EngineBlurImageFilter and takes part in ==/hashCode/description, mirroring native _GaussianBlurImageFilter field for field (including the _boundsString() helper).
• Intentional non-change: getBackendFilter and filterBounds ignore bounds, so every backend falls back to an unbounded blur, like the native Skia backend. Passing bounds through the renderer, so the Impeller-based skwasm variant can render a true bounded blur, is a separate follow-up. Because bounds is final, the existing per-instance backend filter cache (keyed on tile mode) stays valid.
• Tests: test/engine/surface/filters/image_filter_test.dart uses one table-driven test for equality, hash, compose and description, and adds a check that bounded and unbounded blurs report the same filterBounds. It now calls setUpUnitTests() because filterBounds needs a renderer. Zero sigmas print as 0 on dart2js and 0.0 on dart2wasm, so the expected string is built with 0.0.toString().
• Framework test: the framework's web CI runs against a web SDK built from the engine in the same commit, so the unskipped test uses the new dart:ui behavior. I checked it locally against a rebuilt web SDK on Chrome (JS and Wasm), and on the VM.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• 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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