#23365 [BUG][DART] fix nullable nested array item handling

Summary

This PR fixes Dart generator handling of nullable array items, including nested arrays/maps.

Specifically:

• Preserves nullable item markers in Dart type declarations for nested array/map item schemas (e.g. List<String?>, List<List<String?>>, built-value equivalents).
• Fixes native deserialization template logic for nested array/map branches to avoid nullability/type mismatches.
• Fixes nested complex-array mapping to deserialize each element (e) instead of incorrectly reusing the full source array.

Validation

Ran targeted Dart tests successfully:

mise x java@temurin-21 -- ./mvnw -pl modules/openapi-generator -am \
  -Dtest=org.openapitools.codegen.dart.DartModelTest,org.openapitools.codegen.dart.dio.DartDioModelTest \
  -Dsurefire.failIfNoSpecifiedTests=false test

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
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Summary by cubic

Preserves nullable item types in nested arrays/maps and fixes native decoding to keep nulls and correct defaults. Prevents type mismatches and bad casts in Dart2 and DartDio built_value models.

• Bug Fixes

  • Preserve nullable item markers in nested type declarations (e.g., List<String?>, List<List<String?>>, BuiltList<BuiltList<String?>>; also Map<String, T?> and collections of nullable items).
  • Fix native deserialization for nested arrays/maps: map each element e, cast to nullable element types, use typed const defaults, and pass e into listFromJson for complex items.

• Tests/Samples

  • Add tests for nullable items in arrays and nested arrays (incl. complex types) and new nested-array-nullable-items.yaml.
  • Update petstore samples and docs to reflect ? in item types and element-wise mapping.