Unify turn input submission and routing (#38275)

## What changed

- Add `TurnInputRequest` and typed submission results for atomically starting a turn, steering the active turn, or declining input with a specific reason.
- Expose `start_or_steer_turn`, `start_turn_if_idle`, and `steer_turn` on `CodexThread`, and migrate Core consumers to these APIs.
- Make app-server `turn/start` steer an active regular turn and return that turn's ID. Reject incompatible output schemas and non-steerable turns without applying settings or enqueueing input.

## Testing

- Cover concurrent start-or-steer submissions, accepted and rejected settings updates, output-schema compatibility, idle-start rejection, and app-server steering.

GitOrigin-RevId: dd9b5528d76ec650c019e97af420bc13190ea86a

codex-rs/mcp-server/src/message_processor.rs

@@ -11,7 +11,6 @@ use codex_login::AuthManager;
 use codex_login::default_client::USER_AGENT_SUFFIX;
 use codex_login::default_client::get_codex_user_agent;
 use codex_protocol::protocol::SessionSource;
-use codex_protocol::protocol::Submission;
 use rmcp::model::CallToolRequestParams;
 use rmcp::model::CallToolResult;
 use rmcp::model::ClientNotification;
@@ -535,7 +534,7 @@ impl MessageProcessor {
             tracing::warn!("ignoring cancellation without a request id");
             return;
         };
-        // Create a stable string form early for logging and submission id.
+        // Create a stable string form early for logging.
         let request_id_string = request_id.to_string();
 
         // Resolve the thread for the active MCP request.
@@ -559,14 +558,7 @@ impl MessageProcessor {
 
         // Submit interrupt to Codex.
         if let Err(e) = codex_arc
-            .submit_with_id(Submission {
-                id: request_id_string,
-                op: codex_protocol::protocol::Op::Interrupt,
-                client_user_message_id: None,
-                trace: None,
-                parent_turn_id: None,
-                root_turn_id: None,
-            })
+            .submit(codex_protocol::protocol::Op::Interrupt)
             .await
         {
             tracing::error!("Failed to submit interrupt to Codex: {e}");