fix(stdio): drain responses after stdin EOF

[Epochex]

Fixes #2678.

When stdin hits EOF (e.g. python server.py < payload.jsonl > response.jsonl), the client is done sending requests but is still reading stdout for results. On main, BaseSession._receive_loop() closes the write stream as soon as the read stream ends, and Server.run() cancels the handler task group on transport close. That combination can cancel in-flight tool handlers before they respond, dropping the last ools/call responses.

This change lets ServerSession keep the write stream open when the read side closes, so Server.run() can drain already-started handlers and flush their responses before shutdown.

Tests:

• uv run --frozen pytest tests/server/test_cancel_handling.py tests/server/test_stdio.py