feat(mcp): #2228 expose server name on function tools

[IgnazioDS]

Summary

• add a public mcp_server_name field to FunctionTool
• populate that field when MCP tools are converted via MCPUtil.to_function_tool()
• keep the change narrow and typed instead of introducing a general metadata bag
• add focused tests for MCP population, non-MCP default behavior, and copy/replace preservation

Fixes #2228.

Why

Issue #2228 asks for a reliable way to recover which MCP server a converted function tool came from. The maintainer feedback there suggested a specific field such as mcp_server_name: str | None on FunctionTool rather than a generic metadata dict. This patch follows that direction and preserves backward compatibility by making the new field keyword-only with a default of None.

Validation

• uv run ruff check src/agents/tool.py src/agents/mcp/util.py tests/mcp/test_mcp_util.py tests/test_function_tool.py
• PYTHONPATH=src uv run pytest tests/mcp/test_mcp_util.py tests/test_function_tool.py -q
• uv run mypy src/agents/tool.py src/agents/mcp/util.py --disable-error-code import-not-found --disable-error-code unused-ignore

Notes

The local pytest invocation uses PYTHONPATH=src because this workspace clone is not resolving the local agents package via uv run by default, but the test slice itself passes cleanly.

[seratch]

Thanks for sharing this. However, we're going to add the property this way: #2654