Add multi-client agent host integration tests (#304867)

Add 7 new tests covering multi-client scenarios:
- sessionAdded/sessionRemoved notifications broadcast to all clients
- Cross-client message dispatch (client B sends on client A's session)
- Full tool progress updates delivered to both subscribers
- Action scoping: unsubscribed client gets no actions but still gets notifications
- Late subscriber gets current state via snapshot
- Cross-client permission flow (client B confirms tool started by client A)

Co-authored-by: Copilot <[email protected]>