[material_ui] Migrate M3 buttons template to use new gen_defaults

[QuncCccccc]

Work towards flutter/flutter#187899

Fixes flutter/flutter#188405

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[gemini-code-assist]

Code Review

This pull request migrates Material 3 button defaults to be generated from tokens via a new ButtonTemplateM3 template, removing hardcoded default classes from the button implementation files and generating them as part files instead. The review feedback suggests modernizing the generated code by replacing deprecated MaterialStatePropertyAll references with WidgetStatePropertyAll in both the template and its associated tests.

[QuncCccccc]

All comments from gemini-code-assist are related to replace MaterialStatePropertyAll with WidgetStatePropertyAll . I resolved them all with no change because we want to keep the original code as is.