Stabilize multi-agent feature flag (#14622)

- make multi_agent stable and enabled by default
- update feature and tool-spec coverage to match the new default

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/tests/suite/prompt_caching.rs

@@ -177,6 +177,11 @@ async fn prompt_tools_are_consistent_across_requests() -> anyhow::Result<()> {
         "apply_patch",
         "web_search",
         "view_image",
+        "spawn_agent",
+        "send_input",
+        "resume_agent",
+        "wait",
+        "close_agent",
     ]);
     let body0 = req1.single_request().body_json();