## Why
Callers need to tie isolated threads to an explicit lifetime and wait for cleanup even when startup is cancelled or its result is never received.
## What changed
- Add `ThreadManager::start_thread_until` to run an isolated thread until a caller-provided future completes or the thread exits, with a `TaskTracker` covering cleanup and deregistration.
- Retain persistence and session resources across interrupted startup so cleanup can release partially initialized resources or shut down a running session using normal history rules.
- Reject resumed history and startup without explicit session isolation.
## Testing
Add integration tests for cancellation during stalled required MCP initialization, dropping an unconsumed startup result, and owner cancellation that preserves history and parent usability. Also verify that failed duplicate startup leaves the existing thread's writer intact.
GitOrigin-RevId: 47251c1b0c43821ab8f950816c6d341d39cdc823
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