When a depth-2 subagent (Birdie) completes and its parent (Newton) is a
depth-1 subagent, the announce should go to Newton, not bypass to the
grandparent (Jaris).
Previously, isSubagentSessionRunActive(Newton) returned false because
Newton's agent turn completed after spawning Birdie. This triggered the
fallback to grandparent even though Newton's SESSION was still alive and
waiting for child results.
Now we only fallback to grandparent if the parent SESSION is actually
deleted (no sessionId in session store). If the parent session exists,
we inject into it even if the current run has ended — this starts a new
agent turn to process the child result.
Fixes#18037
Test Plan:
- Added regression test: routes to parent when run ended but session alive
- Added regression test: falls back to grandparent only when session deleted
6931ca7035