[material_ui] Migrate M3 DatePicker template to use new gen_defaults

[QuncCccccc]

Work toward flutter/flutter#187899.
Fixes flutter/flutter#188410.

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

Pre-Review Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools.
• I read the [Tree Hygiene] page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran [the auto-formatter].
• I signed the [CLA].
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I [linked to at least one issue that this PR fixes] in the description above.
• I followed [the version and CHANGELOG instructions], using [semantic versioning] and the [repository CHANGELOG style], or I have commented below to indicate which documented exception this PR falls under[^1].
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which [test exemption] this PR falls under[^1].
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request migrates the Material 3 DatePicker defaults to the new token-based generator template (DatePickerTemplateM3), replacing the old template and moving the generated defaults into a separate part file. The generator script and tests have been updated to enable and verify this generation. Review feedback suggests replacing a hardcoded opacity value of 0.60 in the template with a token to improve consistency.

[gemini-code-assist]

The opacity value 0.60 is hardcoded. Other opacity values in this template are derived from tokens (e.g., TokenState.disabledStateLayerOpacity). To improve maintainability and consistency, please consider using a token for this opacity value as well.