* fix(mcp): close thread-scoped MCP sessions on thread deletion
Deleting a thread removed its filesystem data, checkpoints, runs, events,
feedback, metadata and browser session, but never closed its persistent MCP
sessions. MCPSessionPool.close_scope() had no production caller at all, so a
recreated thread id under the same user could reuse stale stateful MCP server
state, and the leaked owner task plus subprocess kept holding memory and file
descriptors.
Add MCPSessionPool.close_thread_scope(user_id, thread_id) and call it from the
reserved deletion path under the same best-effort policy as the neighbouring
cleanups. It matches every incarnation of the thread -- the legacy user:thread
scope and any v2:[user, thread, incarnation] scope -- because a deletion cannot
reliably read the current incarnation: the record may predate incarnation
tracking, and a new incarnation can be minted concurrently. close_scope() is
unchanged for callers that already hold one exact key.
Add mcp_scope_belongs_to_thread() to own the matching rule, and hoist the v2:
prefix into a shared constant so the encoder and matcher cannot drift.
Refs #5188
* test(mcp): bind owner identity in thread cleanup checks
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
