fix: preserve optional string tool arguments

[he-yufeng]

Summary

• keep str | None and other simple scalar union parameters out of JSON pre-parsing
• preserve JSON-looking strings for optional string tool arguments, matching existing plain str behavior
• keep complex unions such as str | list[str] able to decode JSON array inputs

Closes #1873

To verify

• .venv\Scripts\python.exe -m pytest tests\server\mcpserver\test_func_metadata.py -q --basetemp .tmp\pytest -p no:cacheprovider
• .venv\Scripts\python.exe -m ruff check src\mcp\server\mcpserver\utilities\func_metadata.py tests\server\mcpserver\test_func_metadata.py
• .venv\Scripts\python.exe -m ruff format --check src\mcp\server\mcpserver\utilities\func_metadata.py tests\server\mcpserver\test_func_metadata.py
• .venv\Scripts\python.exe -m py_compile src\mcp\server\mcpserver\utilities\func_metadata.py tests\server\mcpserver\test_func_metadata.py
• git diff --check