fix: detect stdin EOF on parent death for stdio transport

[Bortlesboat]

Fixes #2231

Summary

When an MCP server using transport="stdio" has its parent process die, the server process is orphaned and continues running indefinitely. The anyio.wrap_file async iterator may not propagate stdin EOF promptly because it runs readline() in a worker thread.

This adds a background monitor that uses select.poll() to detect POLLHUP on stdin's file descriptor. When the parent process dies and the pipe's write end closes, the monitor cancels the task group, triggering a clean shutdown.

• Only enabled on non-Windows platforms where select.poll() is available
• Polls every 100ms with zero-timeout (non-blocking) checks
• Gracefully degrades (no-op) when stdin has no valid file descriptor (e.g. in tests)
• Existing tests pass unchanged

Test plan

• Existing test_stdio_server passes (uses custom stdin, monitor correctly skipped)
• Manual verification: start server as subprocess, kill parent, confirm server exits

[Bortlesboat]

Pushed a follow-up to address CI failure: pyright flagged g as possibly unbound in ests/server/test_stdio.py.

Commit: e4ae4d2

This removes the unbound reference and should unblock the pre-commit/type check lane.

[Bortlesboat]

Pushed coverage fix (commit 33a96b8): added # pragma: no cover to the os.close(read_fd) lines in both finally blocks. These are cleanup lines in test teardown — anyio task-group cancellation on Python 3.11/3.14 causes coverage to not trace through finally blocks in async tests on certain dependency versions, even though the lines do execute. The pragma resolves the 100% coverage gate failure without removing meaningful coverage from the library code being tested.