[material_ui] Migrate M3 FilterChip template to use new gen_defaults

[QuncCccccc]

Work toward flutter/flutter#187899.
Fixes flutter/flutter#188416.

Depends on #12848. Will wait until #12848 lands first.

Migrates the M3 FilterChip defaults template to the new packages/material_ui/tool/gen_defaults generator.

Pre-Review Checklist

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[gemini-code-assist]

Code Review

This pull request migrates the generation of Material 3 defaults for FilterChip to a token-based template system. It removes the inline _FilterChipDefaultsM3 class from filter_chip.dart and references it as a generated part file. It introduces FilterChipTemplateM3 to generate these defaults, enables the template in the generator script, and adds unit tests to verify the generated output. There are no review comments, and I have no feedback to provide.