…atform directories by default

This change:
- Updates the default analysis_options.yaml templates for new apps, modules, packages, and plugins to exclude the build/ and platform-specific directories (android, ios, web, windows, macos, linux) by default.
- Implements `AnalysisOptionsMigration` to automatically migrate existing projects' analysis_options.yaml files using `package:yaml_edit` to preserve comments and formatting.
- Integrates the migration into `FlutterProject.ensureReadyForPlatformSpecificTooling` so it runs globally for most commands, including `flutter run`, `flutter build`, and `flutter analyze`.
- Adds unit tests for the migration.

Fixes flutter#187728

Addresses feedback from gemini-code-assist bot:
- Makes `excludesToExclude` a const list.
- Uses `YamlEditor.appendToList` instead of `update` to preserve comments inside the exclude list.
- Adds a unit test to verify comment preservation.

Addresses reviewer feedback by adding a unit test to verify that `AnalysisOptionsMigration` correctly handles and merges excludes when the `exclude` list in `analysis_options.yaml` is formatted in flow style (inline brackets, e.g., `[a, b]`).

Thanks to `package:yaml_edit`, this is handled automatically.

…ained

Addresses reviewer feedback by removing `setUp` and group-level state variables in `analysis_options_migration_test.dart` to make each test entirely self-contained, as per the Flutter Style Guide.

To avoid repeating the setup boilerplate in all 9 tests while remaining lint-compliant, this refactoring:
- Introduces a `_TestContext` named record `typedef` to hold the test dependencies.
- Introduces a private helper `_createTestContext()` that returns a fresh record instance for each test.
- Initializes the context locally in each test using `final _TestContext context = _createTestContext();`.
This satisfies the `specify_nonobvious_local_variable_types` lint with a single type annotation per test.

…clude-build

# Conflicts:
#	packages/flutter_tools/lib/src/project.dart