feat(app-server): turn/steer API (#10821)

This PR adds a dedicated `turn/steer` API for appending user input to an
in-flight turn.

## Motivation
Currently, steering in the app is implemented by just calling
`turn/start` while a turn is running. This has some really weird quirks:
- Client gets back a new `turn.id`, even though streamed
events/approvals remained tied to the original active turn ID.
- All the various turn-level override params on `turn/start` do not
apply to the "steer", and would only apply to the next real turn.
- There can also be a race condition where the client thinks the turn is
active but the server has already completed it, so there might be bugs
if the client has baked in some client-specific behavior thinking it's a
steer when in fact the server kicked off a new turn. This is
particularly possible when running a client against a remote app-server.

Having a dedicated `turn/steer` API eliminates all those quirks.

`turn/steer` behavior:
- Requires an active turn on threadId. Returns a JSON-RPC error if there
is no active turn.
- If expectedTurnId is provided, it must match the active turn (more
useful when connecting to a remote app-server).
- Does not emit `turn/started`.
- Does not accept turn overrides (`cwd`, `model`, `sandbox`, etc.) or
`outputSchema` to accurately reflect that these are not applied when
steering.

codex-rs/core/src/codex_thread.rs

@@ -1,5 +1,6 @@
 use crate::agent::AgentStatus;
 use crate::codex::Codex;
+use crate::codex::SteerInputError;
 use crate::error::Result as CodexResult;
 use crate::protocol::Event;
 use crate::protocol::Op;
@@ -9,6 +10,7 @@ use codex_protocol::openai_models::ReasoningEffort;
 use codex_protocol::protocol::AskForApproval;
 use codex_protocol::protocol::SandboxPolicy;
 use codex_protocol::protocol::SessionSource;
+use codex_protocol::user_input::UserInput;
 use std::path::PathBuf;
 use tokio::sync::watch;
 
@@ -45,6 +47,14 @@ impl CodexThread {
         self.codex.submit(op).await
     }
 
+    pub async fn steer_input(
+        &self,
+        input: Vec<UserInput>,
+        expected_turn_id: Option<&str>,
+    ) -> Result<String, SteerInputError> {
+        self.codex.steer_input(input, expected_turn_id).await
+    }
+
     /// Use sparingly: this is intended to be removed soon.
     pub async fn submit_with_id(&self, sub: Submission) -> CodexResult<()> {
         self.codex.submit_with_id(sub).await