Add multi-agent runtime metadata types (#25720)

Stack split from #25708. Original PR intentionally left open. This first
PR adds the multi-agent runtime metadata types and catalog plumbing used
by the rest of the stack.

codex-rs/codex-api/tests/models_integration.rs

@@ -100,6 +100,7 @@ async fn models_client_hits_models_endpoint() {
             supports_search_tool: false,
             auto_review_model_override: None,
             tool_mode: None,
+            multi_agent_version: None,
         }],
     };