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/app-server/tests/common/mcp_process.rs

@@ -62,6 +62,7 @@ use codex_app_server_protocol::ThreadStartParams;
 use codex_app_server_protocol::ThreadUnarchiveParams;
 use codex_app_server_protocol::TurnInterruptParams;
 use codex_app_server_protocol::TurnStartParams;
+use codex_app_server_protocol::TurnSteerParams;
 use codex_core::default_client::CODEX_INTERNAL_ORIGINATOR_OVERRIDE_ENV_VAR;
 use tokio::process::Command;
 
@@ -557,6 +558,15 @@ impl McpProcess {
         self.send_request("turn/interrupt", params).await
     }
 
+    /// Send a `turn/steer` JSON-RPC request (v2).
+    pub async fn send_turn_steer_request(
+        &mut self,
+        params: TurnSteerParams,
+    ) -> anyhow::Result<i64> {
+        let params = Some(serde_json::to_value(params)?);
+        self.send_request("turn/steer", params).await
+    }
+
     /// Send a `review/start` JSON-RPC request (v2).
     pub async fn send_review_start_request(
         &mut self,