[material_ui] Migrate M3 FAB template to use new gen_defaults

[QuncCccccc]

Work toward flutter/flutter#187899.
Fixes flutter/flutter#188415.

This PR migrates the M3 FloatingActionButton defaults template to use the new gen_defaults in material_ui. The generated defaults remain unchanged.

Pre-Review Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
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• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request migrates the Material 3 Floating Action Button (FAB) defaults to a generated implementation using the Material Design token database. Specifically, it removes the hardcoded _FABDefaultsM3 class from floating_action_button.dart and references it as a generated part file. It introduces the FabTemplateM3 template generator, enables it in the generation tool, and adds corresponding unit tests to verify the generated output. There are no review comments, so I have no additional feedback to provide.