Use thread source metadata for Guardian classifiers (#40150)

## What changed

- Mark Guardian classifier requests with `thread_source: guardian_classifier` in turn metadata.
- Remove the classifier-specific `request_kind` and `is_guardian_mode` fields.
- Update sampler and extension tests to expect the new metadata shape.

GitOrigin-RevId: 1654389ea538b7bd879045634816cbace4bd8f5c

codex-rs/ext/guardian-v2/src/async_scorer/extension_tests.rs

@@ -1478,8 +1478,7 @@ async fn contributor_samples_tool_calls_with_the_existing_luna_pool() -> Result<
             "thread_id": classifier_thread_id,
             "guardian_classifier_source_thread_id": thread_id.to_string(),
             "turn_id": "turn-1",
-            "request_kind": "guardian_classifier",
-            "is_guardian_mode": true,
+            "thread_source": "guardian_classifier",
         })
     );
     assert_eq!(request["client_metadata"]["x-openai-subagent"], "guardian");

codex-rs/ext/guardian-v2/src/async_scorer/sampler.rs

@@ -549,8 +549,7 @@ impl LunaSampler {
                 "thread_id": thread_id,
                 "guardian_classifier_source_thread_id": self.config.thread_id,
                 "turn_id": turn_id,
-                "request_kind": "guardian_classifier",
-                "is_guardian_mode": true,
+                "thread_source": "guardian_classifier",
             })
             .to_string();
             request.client_metadata = Some(HashMap::from([

codex-rs/ext/guardian-v2/src/async_scorer/sampler_tests.rs

@@ -80,8 +80,7 @@ fn assert_connection_metadata(server: &responses::WebSocketTestServer) -> Result
                     "thread_id": thread_id,
                     "guardian_classifier_source_thread_id": "thread-1",
                     "turn_id": turn_id,
-                    "request_kind": "guardian_classifier",
-                    "is_guardian_mode": true,
+                    "thread_source": "guardian_classifier",
                 },
             })
         );