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

[Gustl22]

Closes flutter/flutter#158496

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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.

[Gustl22]

It seems, that the goldens fail, although they succeed locally on my Mac. As I am not sure, if we should keep them anyways, I wait until generating on Linux (as I need to set it up then).

[Gustl22]

Can remove goldens (if too costly for CI), but maybe is a good thing to have them in place (more intuitive to see, what's going on).