feat: add thread idle lifecycle hook (#24744)

## Why

Extensions can currently observe thread start, resume, and stop, but
they do not have a lifecycle point for the host to say that immediately
pending thread work has drained. That makes idle follow-up behavior
harder to express as extension-owned logic instead of host-specific
plumbing.

This adds an explicit idle lifecycle hook so an extension can react when
a thread becomes idle while the host keeps ownership of whether any
submitted follow-up input starts a turn, is queued, or is ignored.

## What changed

- Added `ThreadIdleInput` with access to the session-scoped and
thread-scoped extension stores.
- Added a default `on_thread_idle` method to
`ThreadLifecycleContributor`.
- Re-exported `ThreadIdleInput` from the extension API surface.

## Testing

Not run; this only extends the extension API trait surface with a
default hook and exported input type.

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

@@ -18,6 +18,14 @@ pub struct ThreadResumeInput<'a> {
     pub thread_store: &'a ExtensionData,
 }
 
+/// Input supplied when the host has no immediately pending thread work.
+pub struct ThreadIdleInput<'a> {
+    /// Store scoped to the host session runtime.
+    pub session_store: &'a ExtensionData,
+    /// Store scoped to this thread runtime.
+    pub thread_store: &'a ExtensionData,
+}
+
 /// Input supplied when the host stops a thread runtime.
 pub struct ThreadStopInput<'a> {
     /// Store scoped to the host session runtime.