[material_ui] Migrate M3 Checkbox defaults to new gen_defaults

[QuncCccccc]

Work toward flutter/flutter#187899

Fixes flutter/flutter#

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• 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 generation of Material 3 Checkbox defaults to the new token-based template system (CheckboxTemplateM3), moving the generated code from checkbox.dart into a separate part file. It also enables and tests this generation. The reviewer suggested a correction in checkbox_template.dart to use the semantically correct token TokenCheckbox.unselectedOutlineWidth instead of TokenCheckbox.unselectedDisabledOutlineWidth for the error state border side to ensure future-proofing.