[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

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• I linked to at least one issue that this PR fixes in the description above.
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• 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.

[tarrinneal]

Can you add a 6 layer dip version of this into the tests? assuming it works with more layers than 2. (I haven't had a chance to look too hard)

[tarrinneal]

should probably be 26.4.0

[srawlins]

Haha I could! It probably doesn't generate a bonkers amount of code. I'll try for 5-ish layers. (Hopefully a very unrealistic test case, but maybe not.)