Box the session startup future at its API boundary (#41853)

## What changed

- Make `Session::spawn` return a `BoxFuture` while preserving lazy session
  initialization.
- Prevent the startup wrapper from being inlined so callers do not expose its
  concrete future.
- Await the returned future directly at session creation call sites.

GitOrigin-RevId: 5432745aae1c934b16202716a8d49a8553de2018

codex-rs/core/src/codex_delegate.rs

@@ -111,7 +111,7 @@ pub(crate) async fn run_codex_thread_interactive(
     } else {
         Arc::clone(&parent_session.services.extensions)
     };
-    let (session, io) = Box::pin(Session::spawn(SessionSpawnArgs {
+    let (session, io) = Session::spawn(SessionSpawnArgs {
         config,
         allow_provider_model_fallback: false,
         user_instructions,
@@ -158,7 +158,7 @@ pub(crate) async fn run_codex_thread_interactive(
         inherited_multi_agent_version: Some(MultiAgentVersion::Disabled),
         git_enrichment_policy,
         windows_sandbox_proxy_settings_mode,
-    }))
+    })
     .or_cancel(&cancel_token)
     .await??;
     let thread_config = session.thread_config_snapshot().await;

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

@@ -478,7 +478,12 @@ const CYBER_SAFETY_URL: &str = "https://developers.openai.com/codex/concepts/cyb
 
 impl Session {
     /// Spawn and initialize a new session.
-    pub(crate) async fn spawn(args: SessionSpawnArgs) -> CodexResult<(Arc<Self>, SessionIo)> {
+    /// Hide the concrete startup future from callers while keeping initialization lazy.
+    #[inline(never)]
+    pub(crate) fn spawn(
+        args: SessionSpawnArgs,
+    ) -> BoxFuture<'static, CodexResult<(Arc<Self>, SessionIo)>> {
+        Box::pin(async move {
             let parent_trace = match args.parent_trace {
                 Some(trace) => {
                     if codex_otel::context_from_w3c_trace_context(&trace).is_some() {
@@ -500,6 +505,7 @@ impl Session {
             })
             .instrument(thread_spawn_span)
             .await
+        })
     }
 
     async fn spawn_internal(args: SessionSpawnArgs) -> CodexResult<(Arc<Self>, SessionIo)> {

codex-rs/core/src/thread_manager.rs

@@ -1987,7 +1987,7 @@ impl ThreadManagerState {
         } else {
             codex_sandboxing::WindowsSandboxProxySettingsMode::Reconcile
         };
-        let (session, io) = Box::pin(Session::spawn(SessionSpawnArgs {
+        let (session, io) = Session::spawn(SessionSpawnArgs {
             config,
             allow_provider_model_fallback,
             user_instructions,
@@ -2027,7 +2027,7 @@ impl ThreadManagerState {
             inherited_multi_agent_version: multi_agent_version,
             git_enrichment_policy: GitEnrichmentPolicy::Fresh,
             windows_sandbox_proxy_settings_mode,
-        }))
+        })
         .await?;
         // Enable Full Access form input only after session startup so a required MCP server cannot
         // block startup while waiting for form input.