[web] Provide Content-Length header for CanvasKit files in flutter test

[harryterkelsen]

This adds the Content-Length header when serving CanvasKit files in the shelf test server. This prevents shelf from falling back to Transfer-Encoding: chunked, which triggers a bug in Chrome 145's WebAssembly compilation that causes random test hangs. This builds upon PR #189623 which partially mitigated the issue via caching.

Fixes #189275

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests or get an explicit test exemption before merging.

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[gemini-code-assist]

Code Review

This pull request adds the HttpHeaders.contentLengthHeader to the response headers when serving the CanvasKit file in flutter_web_platform.dart. There are no review comments, and I have no feedback to provide.

[eyebrowsoffire]

LGTM