fix(mcp): retain mixed-event bindings for post-run resource release #53

PR #27 expires a relay binding when a selected run outcome is queued. If notify.events also requests thread.resource-released and provider-session.detached occurs afterward, that requested occurrence is missed. Hosted review PRRT_kwDOVfiJHc62itQE classifies this as P2.

Retain a mixed-event binding until the requested thread-level release can be observed, preserving one message per run outcome and per source occurrence. Cover release-before-result and result-before-release, plus restart/retry deduplication. Reuse existing relay state; no additional event migration is allocated.

Tracked under Chris's nonblocking P2 follow-up rule. Default notify does not request resource release and is unaffected. No service update.