Resolve mypy arg-type errors in console tests

[github-actions]

Summary

This PR resolves mypy arg-type errors in console tests by ensuring message dict literals are recognized as the UIMessage TypedDict.

Implementation Details

• Imported UIMessage from oai_coding_agent.console.state in tests/test_rendering.py and tests/test_slash_commands.py.
• Annotated message dict literals (msg) as UIMessage in the affected test functions.

Assumptions

• Using direct TypedDict annotation is preferred over cast() or other workarounds.
• Other mypy errors in unrelated tests will be addressed in separate issues.

Testing

• uv run pytest ✅ All console tests continue to pass.
• uv run mypy . ✅ No arg-type errors in tests/test_rendering.py or tests/test_slash_commands.py.

Considerations

• Alternative approaches considered: using typing.cast to cast dicts to UIMessage, but direct annotation is more concise and clear.

This PR focuses solely on console tests; other test fixes will follow.