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/ext/extension-api/src/contributors/thread_lifecycle.rs

@@ -42,8 +42,21 @@ pub struct ThreadResumeInput<'a> {
     pub thread_store: &'a ExtensionData,
 }
 
+/// Why a thread has no immediately pending work.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum ThreadIdleCause {
+    /// The previous turn completed and automatic follow-up work can run.
+    Completed,
+    /// The user interrupted the previous turn.
+    Interrupted,
+    /// The previous turn ended with a terminal error.
+    Failed,
+}
+
 /// Input supplied when the host has no immediately pending thread work.
 pub struct ThreadIdleInput<'a> {
+    /// Why the thread became idle.
+    pub cause: ThreadIdleCause,
     /// Store scoped to the host session runtime.
     pub session_store: &'a ExtensionData,
     /// Store scoped to this thread runtime.