Add host-reserved V2 child creation APIs (#57586)

## What changed

- Add `CodexThread::create_child` to create an idle child from a live tool invocation using a host-reserved thread ID and controller.
- Add `prepare_child` and `ThreadManager::create_prepared_child` to capture inherited settings and fresh or forked history, then start the child on another manager even after the parent shuts down. Reject detached preparation when live bindings cannot be transferred.
- Add opt-in `HostToolInvocationLookup` to track live calls and reject duplicate active call IDs within a turn. Revoke invocation access when dispatch ends and clean up provisional children when startup fails or is cancelled.

## Testing

Add tests for invocation lifetime and call ID reservation, inherited policy and live bindings, detached startup with fresh and full history, and cancellation cleanup.

GitOrigin-RevId: 71f17ecb4cd7636517e675df59289a8e8c7105b8

codex-rs/core/src/codex_thread.rs

@@ -8,11 +8,15 @@ use crate::elicitation::ElicitationRegistration;
 use crate::session::SessionIo;
 use crate::session::SessionSettingsUpdate;
 use crate::session::Submission;
+use crate::session::emit_subagent_session_started;
 use crate::session::new_submission_id;
+use crate::session::session::AppServerClientMetadata;
 use crate::session::session::Session;
 use crate::session::startup_prewarm::PrewarmInput;
 use crate::session::step_settings::StepSettingsUpdate;
 use crate::thread_startup_metadata::ThreadStartupMetadata;
+use chrono::DateTime;
+use chrono::Utc;
 use codex_diagnostics::Gauge;
 use codex_diagnostics::GaugeGuard;
 use codex_exec_server::SelectedCapabilityRootsStatus;
@@ -45,6 +49,7 @@ use codex_protocol::protocol::MultiAgentVersion;
 use codex_protocol::protocol::Op;
 use codex_protocol::protocol::SandboxPolicy;
 use codex_protocol::protocol::SessionSource;
+use codex_protocol::protocol::SubAgentSource;
 use codex_protocol::protocol::ThreadHistoryMode;
 use codex_protocol::protocol::ThreadMemoryMode;
 use codex_protocol::protocol::ThreadSettingsOverrides;
@@ -289,6 +294,39 @@ impl CodexThread {
         self.session.services.analytics_events_client.is_enabled()
     }
 
+    /// Looks up a live V2 tool invocation on a runtime with [`crate::HostToolInvocationLookup`].
+    pub fn tool_invocation(
+        &self,
+        turn_id: &str,
+        call_id: &str,
+    ) -> CodexResult<crate::ToolInvocationContext> {
+        crate::agent::host::tool_invocation(self, turn_id, call_id)
+    }
+
+    /// Creates an idle V2 child here with the invoking step's environments and live application
+    /// bindings. Compatible children share the parent's exec policy.
+    /// Ending the invocation or dropping this future aborts pending startup.
+    /// [`crate::ThreadManager::create_prepared_child`] defines acceptance and cleanup.
+    pub async fn create_child(
+        &self,
+        invocation: &crate::ToolInvocationContext,
+        request: crate::ThreadCreateRequest,
+        reservation: crate::HostThreadSpawnReservation,
+    ) -> Result<crate::ThreadSpawnResult, crate::ThreadSpawnError> {
+        crate::agent::host::create(self, invocation, request, reservation).await
+    }
+
+    /// Captures inputs from a live invocation for [`crate::ThreadManager::create_prepared_child`].
+    /// The host owns cancellation and snapshot freshness after this returns. Detached preparation
+    /// rejects managed application network, auth, and cloud config bindings.
+    pub async fn prepare_child(
+        &self,
+        invocation: &crate::ToolInvocationContext,
+        request: crate::ThreadCreateRequest,
+    ) -> Result<crate::PreparedThreadSpawn, crate::ThreadSpawnError> {
+        crate::agent::host::prepare(self, invocation, request).await
+    }
+
     /// Returns extension-owned data attached to this thread runtime.
     pub fn thread_extension_data(&self) -> &codex_extension_api::ExtensionData {
         &self.session.services.thread_extension_data
@@ -338,6 +376,29 @@ impl CodexThread {
         self.io.session_loop_termination.clone().await;
     }
 
+    pub(crate) async fn emit_subagent_session_started(
+        &self,
+        client_metadata: AppServerClientMetadata,
+        resumed_created_at: Option<DateTime<Utc>>,
+    ) {
+        let SessionSource::SubAgent(source @ SubAgentSource::ThreadSpawn { .. }) =
+            &self.session_source
+        else {
+            return;
+        };
+        let config = self.config_snapshot().await;
+        emit_subagent_session_started(
+            &self.session.services.analytics_events_client,
+            client_metadata,
+            self.session.session_id(),
+            self.session.thread_id(),
+            config.parent_thread_id,
+            config,
+            source.clone(),
+            resumed_created_at,
+        );
+    }
+
     pub(crate) async fn emit_thread_ready_lifecycle(&self) {
         let contributors = self
             .session