## Why
A message's model-visible item ID may be unknown until the recipient records it. Message lifecycle observers need to capture publication state before that point.
## What changed
Remove `message_id` from `MessageLifecycleInput` and give each publication's `ExtensionData` a fresh UUID key. Stop assigning an item ID when publishing messages for lifecycle observers.
## Testing
Update the agent mailbox test to store a contributor-specific token in `ExtensionData` at publication and retrieve it at consumption, verifying that publication state survives queued delivery and recipient eviction.
GitOrigin-RevId: e254f94994d0976e61cb46ee23297f3248b9a389
## What changed
Add `MessageLifecycleContributor` hooks to capture extension state when local agent control publishes a message and notify contributors when the recipient commits it to history, before sampling.
Carry shared delivery state through submissions, input queues, and mailbox eviction with `PendingAgentMessage`. Notify consumption at most once per delivery, release captured state when consumed or abandoned, and drop pending mail outside queue locks. Preserve the serialized message format without republishing restored history.
## Testing
Extend the mailbox eviction test to verify that publication state survives eviction and retained messages are consumed before the first resumed model sample.
GitOrigin-RevId: 9a1f6832bd48ec646a3fd179485c095e8dcc72cc
## Why
Unread queue-only messages prevented idle agents from unloading, and sending a
message to an evicted agent reloaded its session. Pending mail should not require
an idle recipient to occupy a loaded thread slot.
## What changed
- Retain queue-only mail in an in-memory runtime mailbox when a session unloads.
Queue new messages for locally evicted agents without reloading them or
starting a turn.
- Add `AgentControl::take_mailbox` and `AgentControl::watch_mailbox` so sessions
can consume retained mail and react to host mailbox notifications.
- Preserve message order across eviction, reload, and subsequent follow-ups.
Keep agents with outstanding durable sleep or turn-triggering mail loaded,
and discard retained mail when an agent is removed or its tree shuts down.
## Testing
Add integration coverage for retaining mail across eviction, sending while
unloaded, and ordering messages around a follow-up after reload. Add a host
controller test verifying that mailbox notifications wake `wait_agent` and
that the message is delivered exactly once.
GitOrigin-RevId: 4b55647f9ee9d1aaf3753f0688863bda81617327
Ruslan Nigmatullin