[material_ui] Migrate M3 Card defaults to new gen_defaults

[QuncCccccc]

Work towards flutter/flutter#187899

Fixes flutter/flutter#188406

Pre-Review Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
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[gemini-code-assist]

Code Review

This pull request migrates the Material 3 Card defaults generation to a new template-based generator (CardTemplateM3). It replaces the inline default classes in card.dart with generated part files, enables the generator in gen_defaults.dart, and adds corresponding unit tests in gen_defaults_test.dart. No review comments were provided, so there is no feedback to address.

[elliette]

Thank you!