fix: exit stdio server cleanly on interrupt

[he-yufeng]

Summary

• catch KeyboardInterrupt at the synchronous MCPServer.run() boundary
• keep normal transport errors unchanged
• add a regression test for the stdio branch so Ctrl-C exits without re-raising through AnyIO

To verify

• .\.venv\Scripts\python.exe -m pytest tests\server\mcpserver\test_server.py::TestServer::test_stdio_keyboard_interrupt_exits_cleanly -q
• .\.venv\Scripts\python.exe -m ruff check src\mcp\server\mcpserver\server.py tests\server\mcpserver\test_server.py
• .\.venv\Scripts\python.exe -m ruff format --check src\mcp\server\mcpserver\server.py tests\server\mcpserver\test_server.py
• .\.venv\Scripts\pyright.exe src\mcp\server\mcpserver\server.py tests\server\mcpserver\test_server.py
• git diff --check

Refs #2663