## Why
Multi-agent v2 sub-agents are owned and coordinated by their parent
agent. Allowing an app-server client to start or steer turns on a
spawned child bypasses the multi-agent messaging path and creates a
second, conflicting source of work for that sub-agent.
## What changed
- Reject direct `turn/start` and `turn/steer` requests targeting
multi-agent v2 thread-spawn sub-agents.
- Identify these targets using both the thread's resolved multi-agent
version and its `SubAgentSource::ThreadSpawn` session source, leaving
root threads, v1 agents, and other sub-agent types unchanged.
- Return a consistent invalid-request error before validating or
applying the submitted input.
## Testing
- Added an app-server integration test that spawns a real multi-agent v2
child and verifies that direct `turn/start` and `turn/steer` requests
are rejected.
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