[plugins] Add a flag for tool search. (#15722)

- [x] Add a flag for tool search.

codex-rs/features/src/tests.rs

@@ -120,6 +120,12 @@ fn tool_suggest_is_stable_and_enabled_by_default() {
     assert_eq!(Feature::ToolSuggest.default_enabled(), true);
 }
 
+#[test]
+fn tool_search_is_under_development_and_disabled_by_default() {
+    assert_eq!(Feature::ToolSearch.stage(), Stage::UnderDevelopment);
+    assert_eq!(Feature::ToolSearch.default_enabled(), false);
+}
+
 #[test]
 fn use_linux_sandbox_bwrap_is_a_removed_feature_key() {
     assert_eq!(