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/core/src/state/turn.rs

@@ -1,5 +1,6 @@
 //! Turn-scoped state and active turn metadata scaffolding.
 
+use codex_extension_api::ExtensionData;
 use codex_sandboxing::policy_transforms::merge_permission_profiles;
 use indexmap::IndexMap;
 use std::collections::HashMap;
@@ -75,6 +76,7 @@ pub(crate) struct RunningTask {
     pub(crate) cancellation_token: CancellationToken,
     pub(crate) handle: AbortOnDropHandle<()>,
     pub(crate) turn_context: Arc<TurnContext>,
+    pub(crate) turn_extension_data: Arc<ExtensionData>,
     // Timer recorded when the task drops to capture the full turn duration.
     pub(crate) _timer: Option<codex_otel::Timer>,
 }
@@ -82,6 +84,7 @@ pub(crate) struct RunningTask {
 pub(crate) struct RemovedTask {
     pub(crate) records_turn_token_usage_on_span: bool,
     pub(crate) active_turn_is_empty: bool,
+    pub(crate) turn_extension_data: Arc<ExtensionData>,
 }
 
 impl ActiveTurn {
@@ -97,6 +100,7 @@ impl ActiveTurn {
         Some(RemovedTask {
             records_turn_token_usage_on_span,
             active_turn_is_empty: self.tasks.is_empty(),
+            turn_extension_data: task.turn_extension_data,
         })
     }
 
@@ -120,6 +124,7 @@ pub(crate) struct TurnState {
     pub(crate) tool_calls: u64,
     pub(crate) has_memory_citation: bool,
     pub(crate) token_usage_at_turn_start: TokenUsage,
+    pub(crate) extension_data: Arc<ExtensionData>,
 }
 
 pub(crate) struct PendingRequestPermissions {