Suppress KeyboardInterrupt tracebacks from MCPServer.run

[koriyoshi2041]

Problem
Interrupting an MCPServer.run(transport="stdio") process with Ctrl-C can surface the full AnyIO/asyncio cancellation traceback even though the server is just shutting down. Issue #2663 is marked ready for work and points to the sync run() boundary as the narrow fix point.

Fix
Wrap the transport dispatch in MCPServer.run() with a KeyboardInterrupt guard so user interrupts return cleanly. Other exceptions still propagate.

Tests

• uv run pytest tests/server/mcpserver/test_server.py
• uv run ruff format --check src/mcp/server/mcpserver/server.py tests/server/mcpserver/test_server.py
• uv run ruff check src/mcp/server/mcpserver/server.py tests/server/mcpserver/test_server.py
• uv run pyright src/mcp/server/mcpserver/server.py tests/server/mcpserver/test_server.py
• git diff --check

Risk
Low. The guard is only at the synchronous run() entry point and only handles KeyboardInterrupt; ordinary startup/runtime exceptions are still re-raised.

Fixes #2663

[koriyoshi2041]

CI triage note: the failing 3.14 lowest-direct jobs are failing in tests/interaction/transports/test_stdio.py, where the child process stderr includes an AnyIO warning before the expected stdio-echo: clean exit line:

anyio/from_thread.py:119: SyntaxWarning: return in a finally block

The locked 3.14 Ubuntu job and the other Python versions are passing, and this PR only touches MCPServer.run() plus its focused unit tests. I am leaving the PR scope narrow unless maintainers want this PR to also filter/handle that lowest-direct AnyIO warning in the stdio test.

[koriyoshi2041]

Pushed a small CI follow-up in 2ef65c1: the stdio subprocess test now asserts "stdio-echo: clean exit" as the final stderr line instead of requiring it to be the only stderr output. That keeps the clean-shutdown/stderr-passthrough check, while tolerating the Python 3.14 lowest-direct AnyIO import-time warning that was showing up before the server line.