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

[stuartmorgan-g]

• Updates path_provider_android to use DI to inject the fake Pigeon API implementation, instead of the Dart test generator, following the pattern of newer plugins in the repo.
• Does the same for path_provider_foundation, and also replaces the use of mockito with a trivial fake, to simplify test maintenance.

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

[stuartmorgan-g]

CHANGELOG/version overrides: While technically there is a 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.

[gemini-code-assist]

Code Review

This pull request refactors the tests for path_provider_android and path_provider_foundation to no longer use Pigeon's Dart test generator. Instead, it uses dependency injection to provide a fake API implementation for testing. For path_provider_foundation, it also removes the dependency on mockito in favor of a simpler manual fake. The changes align with modern practices in the repository and simplify test maintenance. I have one suggestion to improve the test clarity in path_provider_android by adopting the more explicit fake implementation pattern used in path_provider_foundation.