Expose connector candidates in external agent detection (#36218)

## What changed

- Add a `connectors` array to `ExternalAgentConfigDetectResponse` with each candidate's normalized name, detected session count, and detection source.
- Define detection sources for remote MCP server configuration and session tool use, and export the corresponding JSON Schema and TypeScript types.
- Default `connectors` to an empty array when deserializing responses from older servers.

## Testing

- Add a protocol test covering deserialization when `connectors` is omitted.

GitOrigin-RevId: b65d76791099385226ed54411f4c1ef902be43d8

codex-rs/app-server-protocol/src/protocol/v2/tests.rs

@@ -72,6 +72,22 @@ fn test_absolute_path() -> AbsolutePathBuf {
     absolute_path("readable")
 }
 
+#[test]
+fn external_agent_config_detect_response_defaults_connectors_for_older_servers() {
+    let response = serde_json::from_value::<ExternalAgentConfigDetectResponse>(json!({
+        "items": [],
+    }))
+    .expect("older detect response should deserialize");
+
+    assert_eq!(
+        response,
+        ExternalAgentConfigDetectResponse {
+            items: Vec::new(),
+            connectors: Vec::new(),
+        }
+    );
+}
+
 #[test]
 fn thread_background_terminals_list_response_round_trips_foreign_paths() {
     for (uri, expected_cwd) in [