[pigeon] Support lists-and-maps of lists-and-maps

[srawlins]

Fixes flutter/flutter#116117

This PR is massive because of the added test data. 😬 But the source changes, in lib/src/dart/dart_generator.dart, are pretty minimal.

Please let me know if I added too many new test data, or if I need more.

The strategy is summed up in the doc comment to the _castCall method:

• Given a simple type, like List<int>, the returned code is a simple call to List.cast: cast<int>().
• Given a deep List type, like List<List<int>>, the returned code uses List.map and List.cast: map((e) => (e! as List<Object?>).cast<int>()).toList().
• Given a deep Map type, like Map<int, List<int>>, the returned code uses Map.map and Map.cast: map((k, v) => MapEntry(k! as int, (v! as List<Object?>).cast<int>())).

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• 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 followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under[^1].
• 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 under[^1].
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request updates the Pigeon Dart generator to support nested List and Map types by implementing a recursive casting mechanism. It adds nested collection fields such as boolListList, boolMapList, and boolListMap to the core test pigeons and introduces a new echoNonNullBoolListList method to the host integration API. The changes encompass updates to generated code across all supported platforms and the addition of integration tests. Feedback indicates that the objectVersion in the macOS Xcode project file was downgraded, which appears to be an unintentional change that should be reverted.