Add durable user-message queue dispatch (#37204)

## What changed

- Add a storage-neutral queue extension for listing, adding, editing, reordering, deleting, and explicitly starting queued user messages.
- Dispatch queued messages in FIFO order when a thread becomes idle after a completed turn, while leaving them queued after interruptions or failures.
- Wait for user input to be persisted before removing its queue item, and report hook rejection, task termination, and persistence failures distinctly.
- Snapshot local image and audio attachments into validated data URLs before storing them, enforce input limits, and emit `ThreadQueueChanged` events after mutations.

## Testing

- Cover queue mutations, automatic and explicit dispatch, interruption and failure behavior, hook rejection, invalid records, input limits, and local attachment snapshotting.

GitOrigin-RevId: 14f39d7c8cbaccb767a6da84b82cced0edfcb4b9

codex-rs/core/src/lib.rs

@@ -35,6 +35,7 @@ pub use codex_thread::TryStartTurnIfIdleError;
 pub use codex_thread::TryStartTurnIfIdleRejectionReason;
 pub use session::turn_context::TurnContext;
 pub use user_message_admission::UserMessageAdmission;
+pub use user_message_admission::UserMessageAdmissionError;
 mod agent;
 mod agent_communication;
 mod attestation;