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/mod.rs

@@ -422,6 +422,7 @@ pub(crate) struct SessionSpawnArgs {
     pub(crate) environment_selections: Vec<TurnEnvironmentSelection>,
     pub(crate) thread_extension_init: ExtensionDataInit,
     pub(crate) client_mcp_extensions: ClientMcpExtensions,
+    pub(crate) reserved_thread_id: Option<ThreadId>,
     pub(crate) analytics_events_client: Option<AnalyticsEventsClient>,
     pub(crate) thread_store: Arc<dyn ThreadStore>,
     pub(crate) attestation_provider: Option<Arc<dyn AttestationProvider>>,
@@ -514,6 +515,7 @@ impl Session {
             environment_selections,
             thread_extension_init,
             client_mcp_extensions,
+            reserved_thread_id,
             analytics_events_client,
             thread_store,
             attestation_provider,
@@ -742,6 +744,7 @@ impl Session {
             thread_extension_init,
             client_mcp_extensions,
             agent_control,
+            reserved_thread_id,
             environment_manager,
             inherited_environments,
             analytics_events_client,