Fixes #2678.

Summary

• keep the stdio write side open after redirected stdin EOF so already accepted request handlers can finish and flush responses
• bound stdio EOF draining with a 5s timeout while preserving the existing immediate-cancel behavior for other transport closes
• add a regression test that drives an MCPServer through redirected stdin and asserts response ids 0, 1, and 2 are all written before process exit

Tests

• python -m ruff check src/mcp/shared/session.py src/mcp/server/session.py src/mcp/server/lowlevel/server.py src/mcp/server/mcpserver/server.py tests/issues/test_2678_stdio_eof_drain.py
• python -m ruff format --check src/mcp/shared/session.py src/mcp/server/session.py src/mcp/server/lowlevel/server.py src/mcp/server/mcpserver/server.py tests/issues/test_2678_stdio_eof_drain.py
• python -m pyright src/mcp/shared/session.py src/mcp/server/session.py src/mcp/server/lowlevel/server.py src/mcp/server/mcpserver/server.py tests/issues/test_2678_stdio_eof_drain.py
• python -m pytest tests/issues/test_2678_stdio_eof_drain.py tests/server/test_cancel_handling.py tests/server/test_session.py -q