[file_selector] Remove use of Pigeon's Dart test generator

[stuartmorgan-g]

Updates file_selector_ios, _macos, and _windows tests to use the same structure as file_selector_linux already uses:

• Removes the use of Pigeon's Dart test generator in favor of DI'ing an implementation of the actual generated Dart API.
• Removes the use of mockito in favor of a fake, since the fake is extremely simple to maintain and it makes the tests simpler.

Part of flutter/flutter#159886

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• 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 updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• 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 under¹.
• All existing and new tests are passing.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩² ↩³

[gemini-code-assist]

Code Review

This pull request refactors the tests for file_selector_ios, file_selector_macos, and file_selector_windows to remove the dependency on Pigeon's Dart test generator and the mockito package. This is achieved by introducing dependency injection for the platform-specific FileSelectorApi and using a simple fake implementation in tests. This change simplifies the test setup, reduces dependencies, and removes generated test files, improving the overall maintainability of the codebase. The changes are consistent across all affected packages and I've noted one opportunity for improvement.

[stuartmorgan-g]

CHANGELOG/version overrides: While technically there is production code change here, it's just adding an optional, visibible-only-for-tests parameter, and moving one ivar initialization from the declaration to the constructor, so it's pretty clearly a no-op.

[stuartmorgan-g]

This wasn't being used, it was just copypasta from some other platform, which is why there are no other changes to file_selector_linux.