Remove Guardian subagent-spawner plumbing (#45491)

## What changed

- Remove `AgentSpawner` and `AgentSpawnFuture` from the extension API, along with the Guardian wrapper, thread lifecycle context, and app-server injection plumbing.
- Define `InternalSessionSpawnFuture` directly as a boxed future instead of aliasing `AgentSpawnFuture`.
- Raise the workspace `rustls` minimum version to `0.23.45`.

GitOrigin-RevId: b7319dee41bfb869479afeb7555a6f050c4d00a5

codex-rs/ext/extension-api/src/capabilities/agent.rs

@@ -1,38 +0,0 @@
-use std::future::Future;
-use std::pin::Pin;
-
-use codex_protocol::ThreadId;
-
-/// Future returned by one injected subagent-spawn helper.
-pub type AgentSpawnFuture<'a, T, E> = Pin<Box<dyn Future<Output = Result<T, E>> + Send + 'a>>;
-
-/// Constructor-injected host helper for extensions that need to spawn subagents.
-///
-/// The extension owns the request shape and resulting handle types. The host
-/// provides the implementation when it constructs the extension.
-pub trait AgentSpawner<R>: Send + Sync {
-    type Spawned;
-    type Error;
-
-    fn spawn_subagent<'a>(
-        &'a self,
-        forked_from_thread_id: ThreadId,
-        request: R,
-    ) -> AgentSpawnFuture<'a, Self::Spawned, Self::Error>;
-}
-
-impl<R, S, E, F> AgentSpawner<R> for F
-where
-    F: Fn(ThreadId, R) -> AgentSpawnFuture<'static, S, E> + Send + Sync,
-{
-    type Spawned = S;
-    type Error = E;
-
-    fn spawn_subagent<'a>(
-        &'a self,
-        forked_from_thread_id: ThreadId,
-        request: R,
-    ) -> AgentSpawnFuture<'a, Self::Spawned, Self::Error> {
-        self(forked_from_thread_id, request)
-    }
-}

codex-rs/ext/extension-api/src/capabilities/internal_session.rs

@@ -1,9 +1,11 @@
+use std::future::Future;
+use std::pin::Pin;
+
 use codex_protocol::ThreadId;
 
-use super::agent::AgentSpawnFuture;
-
 /// Future returned by one host-owned internal-session spawning request.
-pub type InternalSessionSpawnFuture<'a, T, E> = AgentSpawnFuture<'a, T, E>;
+pub type InternalSessionSpawnFuture<'a, T, E> =
+    Pin<Box<dyn Future<Output = Result<T, E>> + Send + 'a>>;
 
 /// Constructor-injected host helper for extensions that need private internal sessions.
 ///

codex-rs/ext/extension-api/src/capabilities/mod.rs

@@ -1,12 +1,9 @@
-mod agent;
 mod conversation_history;
 mod events;
 mod internal_session;
 mod metrics;
 mod response_items;
 
-pub use agent::AgentSpawnFuture;
-pub use agent::AgentSpawner;
 pub use conversation_history::ConversationHistorySnapshot;
 pub use events::ExtensionEventSink;
 pub use events::ExtensionWarning;