No cancellation/cleanup of plugin subprocesses on client disconnect

[cursor]

Summary

Plugin subprocesses are not cancelled/terminated when a client disconnects or a request is cancelled, potentially leaving orphan processes.

Evidence

• execute_plugin_tool awaits process.communicate() with no cancellation handling.
• No link between request lifecycle and subprocess lifecycle.

Impact

• Orphaned processes and resource leaks under client disconnects or timeouts.

Proposed direction

• Tie subprocess lifetime to request/task cancellation; on cancellation, send terminate then kill after grace period.
• Ensure cleanup paths log and surface structured errors to the client.

Acceptance Criteria

• Cancelling a request or disconnecting closes associated plugin processes promptly.