Support metadata staging for reserved thread IDs (#38819)

## Why

Callers need to associate host-owned state with a thread before Core starts it.

## What changed

- Add `ThreadManager::reserve_thread_id` and allow new threads to start with the reserved ID. Reject reserved IDs when resuming an existing thread.
- Let thread stores stage metadata for a reserved ID. The local store merges staged values into the first successful metadata update and then removes them.
- Clear staged metadata when an unmaterialized thread shuts down or is discarded, and require the state database for staging.

## Testing

- Cover reserved ID generation and resume validation.
- Cover metadata persistence, merging, rollout-compatible updates, cleanup, and invalid staging requests.

GitOrigin-RevId: 3f9bc48f0f7764011cc90ea01349e497dda0a9e3

codex-rs/core/src/session/session.rs

@@ -605,6 +605,7 @@ impl Session {
         mut thread_extension_init: ExtensionDataInit,
         client_mcp_extensions: ClientMcpExtensions,
         agent_control: AgentControl,
+        reserved_thread_id: Option<ThreadId>,
         environment_manager: Arc<EnvironmentManager>,
         inherited_environments: Option<TurnEnvironmentSnapshot>,
         analytics_events_client: Option<AnalyticsEventsClient>,
@@ -663,11 +664,20 @@ impl Session {
         let multi_agent_version = multi_agent_version.map(OnceLock::from).unwrap_or_default();
         let initial_multi_agent_version = multi_agent_version.get().copied();
 
-        let thread_id = match &initial_history {
-            InitialHistory::New | InitialHistory::Cleared | InitialHistory::Forked(_) => {
+        let thread_id = match (&initial_history, reserved_thread_id) {
+            (
+                InitialHistory::New | InitialHistory::Cleared | InitialHistory::Forked(_),
+                Some(thread_id),
+            ) => thread_id,
+            (InitialHistory::New | InitialHistory::Cleared | InitialHistory::Forked(_), None) => {
                 agent_control.generate_thread_id()
             }
-            InitialHistory::Resumed(resumed_history) => resumed_history.conversation_id,
+            (InitialHistory::Resumed(resumed_history), None) => resumed_history.conversation_id,
+            (InitialHistory::Resumed(_), Some(_)) => {
+                return Err(anyhow::anyhow!(
+                    "reserved thread ID cannot be used when resuming a thread"
+                ));
+            }
         };
         let resumed_session_id = match &initial_history {
             InitialHistory::Resumed(resumed) => {