Fix abort turn handling

[shayne]

fix: respond to tool calls on turn aborted

Issue:

• tools/call requests can hang after a user aborts a turn, leaving the MCP request open
• subsequent prompts may be ignored because the prior request still owns the response

Reason:

• the MCP tool runner ignored EventMsg::TurnAborted, so it never sent a response or cleared the request map

Solution:

• send a CallToolResult marked isError on TurnAborted, remove the request from the map, and exit the event loop
• add a regression test that cancels an in-flight tool call and asserts a response is returned
• add a test helper to send notifications/cancelled

Test:

• cargo test -p codex-mcp-server -- test_tool_call_returns_on_turn_aborted