[material_ui] Migrate AppBar M3 template to use new gen_defaults script

[QuncCccccc]

Will land #12209 first.

Fixes flutter/flutter#188397

Initially gen_defaults script and its templates were used to handle JSON tokens. After we fix the token pipeline and make it possible to get M3E tokens, we use Dart constants and re-created the gen_defaults in material_ui. After decoupling, the old gen_defaults will be deprecated soon.

This PR is to:

• create a template for M3 AppBar so we can use new script to generate M3 defaults.
• move the generated code to generated/ folder so all M3 and M3E generated code will be in the same location.

Pre-Review Checklist

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

[gemini-code-assist]

Code Review

This pull request extracts the generated Material 3 AppBar defaults into a separate file, app_bar_m3_defaults.g.dart, and introduces an AppBarTemplate class to generate this file automatically. A unit test has also been added to verify the generated defaults. Feedback suggests removing an obsolete // ignore: unused_local_variable comment in gen_defaults.dart since the verbose variable is now utilized.

[elliette]

Haven't done a thorough review, just a few quick questions.

[gemini-code-assist]

Code Review

This pull request migrates the generation of Material 3 AppBar defaults to a new template-based generator. It removes the hardcoded defaults and scroll-under configurations from 'app_bar.dart' and moves them to a generated file 'app_bar_defaults_m3.g.dart'. A new 'AppBarTemplateM3' class is introduced to generate these defaults from token data, the 'gen_defaults.dart' tool is updated to invoke this template, and corresponding unit tests are added in 'gen_defaults_test.dart'. There are no review comments, and I have no feedback to provide.

[elliette]

Looks great, thank you!