[vector_graphics] Respect BoxFit parameter with viewbox (closes #158496)

[Gustl22]

Closes flutter/flutter#158496

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
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[gemini-code-assist]

Code Review

This pull request fixes an issue where BoxFit was not being respected for vector graphics with a viewBox. The change refactors the sizing logic in _VectorGraphicWidgetState to correctly use FittedBox. The PR also renames an internal variable for clarity and adds extensive new tests, including golden file tests, to verify the fix. The changes are logical and well-tested. I've included a few minor suggestions to align with Dart's style guide.