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

[stuartmorgan-g]

Updates image_picker_ios to use the same test structure that image_picker_android 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 the old method channel logger (which was adapted in place during the Pigeon conversion to minimize test changes in the PR that changed the implementation; the fact that it was still being used was tech debt), in favor of structured storage of passed arguments that can be queried directly. This makes tests much shorter and easier to understand and maintain, because if we want to check one parameter in a targeted test, we don't have to assert the entire argument dictionary.
• Adds some missing test functionality (e.g., we didn't have tests of any of the non-null return value handling), and removes some duplicate testing (e.g., every possible combination of null and non-null width, hight, and image quality in on giant test).

Also does minor cleanup of image_picker_android; this conversion had already been done there, but it was still generating an unused dartTestOut.

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 is a significant improvement, refactoring the image_picker_ios tests to remove the dependency on Pigeon's Dart test generator and align with the image_picker_android implementation. The introduction of dependency injection with a fake API mock makes the tests much cleaner, more focused, and easier to maintain. The removal of the old method call logger and the addition of more granular tests are excellent changes. My review includes a few minor suggestions to make the new tests even more precise by removing or adjusting assertions that check for parameters not relevant to the specific method being tested.