fix(kokoros): implement face_verify and face_analyze trait stubs (#9499)

The backend.proto was updated to add FaceVerify and FaceAnalyze RPCs
(face detection support), but the Rust KokorosService was never updated
to match the regenerated tonic trait, breaking compilation with E0046:

    not all trait items implemented, missing: `face_verify`, `face_analyze`

Stubs both methods as unimplemented, matching the pattern used for the
other RPCs Kokoros does not support.

Assisted-by: Claude:claude-opus-4-7 [Claude Code]

backend/rust/kokoros/src/service.rs

@@ -372,6 +372,20 @@ impl Backend for KokorosService {
         Err(Status::unimplemented("Not supported"))
     }
 
+    async fn face_verify(
+        &self,
+        _: Request<backend::FaceVerifyRequest>,
+    ) -> Result<Response<backend::FaceVerifyResponse>, Status> {
+        Err(Status::unimplemented("Not supported"))
+    }
+
+    async fn face_analyze(
+        &self,
+        _: Request<backend::FaceAnalyzeRequest>,
+    ) -> Result<Response<backend::FaceAnalyzeResponse>, Status> {
+        Err(Status::unimplemented("Not supported"))
+    }
+
     async fn stores_set(
         &self,
         _: Request<backend::StoresSetOptions>,