Fix generic type mismatch caused by missing type argument.

[SeifShaheen]

This pull request introduces enhancements to the type inference and subtyping logic for extension types and Fixes #62242 , adds new targeted test cases for extension type inference and subtyping. The most important changes are grouped below by theme.

Type Inference and Subtyping Improvements

• Updated TypeConstraintGenerator in type_analyzer_operations.dart to handle subtype constraint generation for extension type declarations, ensuring correct erasure and comparison logic.
• Modified TypeSchemaEnvironment in type_schema_environment.dart to add a fallback subtype check for extension type variables, allowing legacy/undetermined extension type parameters to be assignable to their non-nullable equivalents when the bound is valid.

Extension Type Test Coverage

• Added a new test case issue_inference.dart to verify inference and behavior for classes and extensions using extension types as type parameters.
• Added a new test case test_rep_subtyping.dart to check subtyping relationships between extension types and their representation types.

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[copybara-service]

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[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/471322 has been updated with the latest commits from this pull request.

[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/471322 has been updated with the latest commits from this pull request.

[mraleph]

@SeifShaheen Thank you for your contribution, unfortunately it is fixing things in the wrong way - an engineer on the front-end team will submit the proper fix instead. Type system and inference rules are rather complex so next time consider choosing a simpler target as your first contribution.