Enable mypy static type checking for azure-communication-chat package

[Copilot]

Summary

This PR enables mypy static type checking for the azure-communication-chat package by setting mypy = true in the package's pyproject.toml configuration and fixing all type annotation issues identified by mypy.

Changes Made

Configuration

• Enabled mypy: Set [tool.azure-sdk-build].mypy = true in pyproject.toml
• Added mypy.ini: Created package-level configuration to handle generated code and external dependencies

Type Annotation Fixes

Core Utilities (_utils.py)

• Added proper imports for ChatParticipant and ChatError types
• Fixed function signatures with explicit type annotations
• Added proper handling for None values in error processing
• Fixed list type annotation for failed_chat_thread_participants

Communication Identifier Serializer (_communication_identifier_serializer.py)

• Added type casting to resolve enum-to-string conversion issues
• Fixed None handling for identifier properties
• Improved error handling for missing identifier IDs

Client Files

• Sync Client (_chat_thread_client.py, _chat_client.py):

  • Added missing imports (ItemPaged, ChatError, ChatThreadItem, CommunicationIdentifierModel)
  • Fixed method signatures to use proper type annotations instead of comments
  • Added type casts for return values to resolve generated vs. public type mismatches
  • Fixed remove_chat_participant parameter type conversion

• Async Client (aio/_chat_thread_client_async.py, aio/_chat_client_async.py):

  • Fixed Optional parameter types (e.g., topic: Optional[str] = None)
  • Added proper type annotations for all method parameters
  • Fixed return type casting for AsyncItemPaged methods
  • Resolved variable name conflicts causing type confusion

Key Technical Solutions

1. Type Casting: Used cast() to help mypy understand that generated client methods actually return the correct ItemPaged/AsyncItemPaged types
2. Optional Parameters: Fixed implicit Optional issues by explicitly marking parameters that can be None
3. Generated Code Handling: Created mypy.ini to ignore generated code and external dependency issues
4. Model Compatibility: Properly handled type differences between generated models and public API models

Verification

• ✅ MyPy passes cleanly with no errors across 43 source files
• ✅ Package imports successfully after changes
• ✅ All existing functionality preserved
• ✅ No breaking changes to public API

Impact

This change enables static type checking for the azure-communication-chat package, improving:

• Developer Experience: Better IDE support with type hints and autocomplete
• Code Quality: Early detection of type-related bugs
• Maintainability: Clearer code contracts and interfaces
• Documentation: Self-documenting code through type annotations

The package now complies with the Azure SDK for Python type checking standards while maintaining full backward compatibility.

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