## Why
`start_or_steer_turn` rejected agent messages, preventing callers from submitting follow-up work through the turn input API.
## What changed
Route `TurnInput::InterAgentCommunication` through native agent message delivery and return `TurnInputSubmission::Accepted`, without implying an immediate turn ID. These submissions request work regardless of `trigger_turn`, retain the recipient's settings, and forward turn-start options.
Wrap plaintext messages with agent context while preserving encrypted content. Reject submissions to idle recipients when the host is draining.
## Testing
Add scenario tests and snapshots covering queued-message ordering, encrypted follow-ups, preemption at reasoning boundaries, and follow-ups arriving before or after a final answer. Extend drain-gate coverage to agent submissions with parent-turn lineage.
GitOrigin-RevId: 92d49465f97bb366babbf1b1bc18a1e8a6c6ed1c
Won Park
05c08d7655