fix: MCP session not cleaned up gracefully on timeout

[yoavkatz]

Summary

When an agent session times out, the MCP sidecar process is killed before the harness has a chance to call MCP.DeleteSession, resulting in an error during cleanup and a non-graceful session teardown.

Observed Behavior

In trace tr-6635ef947049886b3b143e621b508cab (TAU2, tool_calling, Qwen3.6-35B-A3B), the MCP.DeleteSession span errors with:

RuntimeError: Session deletion failed: Session deletion failed:
  {'error': 'Error closing session 5788bd81-33f1-471b-9ec2-c75a9110bd02:
   Service at http://127.0.0.1:59685 is no longer alive'}

The session output is "Session completed (timeout)", meaning the MCP process was already dead when the harness tried to tear it down.

The Agent.Session span inherits this as its top-level error status, making the trace appear as ERROR even though the primary failure was the agent timeout — not a crash.

Impact

• Session cleanup leaves resources in an indeterminate state.
• The Agent.Session span is marked ERROR due to the cleanup failure, conflating two distinct failure modes (agent timeout vs. infrastructure error) in trace analysis.
• A second MCP.DeleteSession call is attempted and silently succeeds (status UNSET), indicating a retry path exists but the first attempt's error still propagates up.

Root Cause

The MCP sidecar at the local address (127.0.0.1:<port>) is terminated as part of the timeout handling before MCP.DeleteSession is invoked. The delete call then hits a dead socket.

Expected Behavior

• MCP.DeleteSession should check whether the session/process is still alive before attempting to close it, and handle the "already dead" case as a clean no-op (or at most a warning), not an error.
• Alternatively, the timeout handler should ensure MCP.DeleteSession is called (or skipped safely) before killing the sidecar process.
• The Agent.Session span's error status should reflect the agent-level outcome (timeout), not a cleanup side-effect.

Trace Reference

• Trace ID: tr-6635ef947049886b3b143e621b508cab
• Benchmark: tau2
• Agent: tool_calling
• Model: openai/Qwen3.6-35B-A3B
• Session ID: 5788bd81-33f1-471b-9ec2-c75a9110bd02