feat: add turn lifecycle contributors (#22480)

## Why

Extensions can already contribute prompt, tool, turn-item, and
thread-lifecycle behavior, but there was no explicit host-owned hook for
per-turn setup and cleanup. That makes extension-private turn state
awkward: an extension either has to stash it outside the turn lifecycle
or depend on core runtime objects.

This adds a small turn lifecycle boundary. Extensions receive stable
identifiers plus the existing session, thread, and turn `ExtensionData`
stores, while core keeps owning task scheduling, cancellation, and turn
teardown.

## What Changed

- Added `TurnLifecycleContributor` with `on_turn_start`, `on_turn_stop`,
and `on_turn_abort` callbacks in `codex-rs/ext/extension-api`.
- Added typed `TurnStartInput`, `TurnStopInput`, and `TurnAbortInput`
payloads that expose `thread_id`, `turn_id`, `session_store`,
`thread_store`, and `turn_store`.
- Registered and re-exported turn lifecycle contributors through
`ExtensionRegistry` and `ExtensionRegistryBuilder`.
- Wired `Session` to emit turn start, stop, and abort callbacks from the
existing turn/task lifecycle paths.
- Carried the turn-scoped `ExtensionData` through `RunningTask` and
`RemovedTask` so stop/abort callbacks receive the same turn store
created at turn start.

## Verification

- Not run locally.

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

@@ -0,0 +1,48 @@
+use codex_protocol::ThreadId;
+use codex_protocol::protocol::TurnAbortReason;
+
+use crate::ExtensionData;
+
+/// Input supplied when the host starts a turn.
+pub struct TurnStartInput<'a> {
+    /// Identifier for the thread containing this turn.
+    pub thread_id: ThreadId,
+    /// Identifier for the turn that is starting.
+    pub turn_id: &'a str,
+    /// Store scoped to the host session runtime.
+    pub session_store: &'a ExtensionData,
+    /// Store scoped to this thread runtime.
+    pub thread_store: &'a ExtensionData,
+    /// Store scoped to this turn runtime.
+    pub turn_store: &'a ExtensionData,
+}
+
+/// Input supplied when the host completes a turn.
+pub struct TurnStopInput<'a> {
+    /// Identifier for the thread containing this turn.
+    pub thread_id: ThreadId,
+    /// Identifier for the turn that is stopping.
+    pub turn_id: &'a str,
+    /// Store scoped to the host session runtime.
+    pub session_store: &'a ExtensionData,
+    /// Store scoped to this thread runtime.
+    pub thread_store: &'a ExtensionData,
+    /// Store scoped to this turn runtime.
+    pub turn_store: &'a ExtensionData,
+}
+
+/// Input supplied when the host aborts a turn.
+pub struct TurnAbortInput<'a> {
+    /// Identifier for the thread containing this turn.
+    pub thread_id: ThreadId,
+    /// Identifier for the turn that is aborting.
+    pub turn_id: &'a str,
+    /// Reason the host aborted the turn.
+    pub reason: TurnAbortReason,
+    /// Store scoped to the host session runtime.
+    pub session_store: &'a ExtensionData,
+    /// Store scoped to this thread runtime.
+    pub thread_store: &'a ExtensionData,
+    /// Store scoped to this turn runtime.
+    pub turn_store: &'a ExtensionData,
+}