fix: support PEP 604 structured output unions

[he-yufeng]

Summary

• wrap FastMCP structured output annotations with Pydantic's explicit (annotation, ...) field form
• add regression coverage for dict | list | str return annotations

Fixes#2591.

To verify

• uv run pytest tests\server\mcpserver\test_func_metadata.py -q --basetemp .tmp\pytest -p no:cacheprovider
• uv run ruff check src\mcp\server\mcpserver\utilities\func_metadata.py tests\server\mcpserver\test_func_metadata.py
• uv run ruff format --check src\mcp\server\mcpserver\utilities\func_metadata.py tests\server\mcpserver\test_func_metadata.py
• git diff --check

[he-yufeng]

Pushed a follow-up for the pre-commit pyright failure: the regression helper now uses typed dict/list generics instead of bare dict | list | str. Revalidated locally with:

• uv run pytest tests\server\mcpserver\test_func_metadata.py -q --basetemp .tmp\pytest -p no:cacheprovider
• uv run ruff check src\mcp\server\mcpserver\utilities\func_metadata.py tests\server\mcpserver\test_func_metadata.py
• uv run ruff format --check src\mcp\server\mcpserver\utilities\func_metadata.py tests\server\mcpserver\test_func_metadata.py
• uv run pyright tests\server\mcpserver\test_func_metadata.py
• git diff --check

[Swcmb]

Thanks for working on this fix! I'm the original reporter of #2591 — great to see the community jumping on this so quickly.

I noticed that #2599 (by @MukundaKatta) has already been merged to address the same PEP 604 union issue. It might be worth coordinating with that fix to avoid conflicts.

Appreciate the effort! 🙌

[he-yufeng]

Thanks for pointing this out. I rechecked #2599; GitHub shows it as closed without being merged, so this PR still appears to be the open candidate for #2591.

The branch is currently all-green. I'll leave the scope as-is unless maintainers prefer a different shape.

[he-yufeng]

Closing this during backlog cleanup. The python-sdk queue has too many open PRs, and this older/no-human-signal item is lower priority than the remaining PRs that have recent validation, review discussion, or clearer current value. I can reopen or refresh it if maintainers want this specific change.