ci: wire up MLX unit tests for PR runs (#17022)

* ci: wire up MLX unit tests for PR runs

Download the latest Darwin release payload matching the current MLX and MLX-C revisions so macOS PR tests can exercise MLX without rebuilding it. If no matching release exists after a pin bump, leave MLX tests skipped until the next release.

Add whole-tree race coverage and smoke-run committed benchmarks. Verify generated UI types, and stabilize tests exposed by the broader CI coverage.

* review comments

* mlx: run tests on one pinned worker

Keep MLX tests and benchmarks on a shared pinned thread while preserving Fatal, Skip, and Cleanup semantics. Also clean stale CI payloads and ensure updater workers shut down cleanly.

* addres comments

x/transfer/transfer_test.go

@@ -1680,6 +1680,10 @@ func BenchmarkUploadThroughput(b *testing.B) {
 		case http.MethodPost:
 			w.Header().Set("Location", fmt.Sprintf("%s/v2/library/_/blobs/uploads/1", serverURL))
 			w.WriteHeader(http.StatusAccepted)
+		case http.MethodPatch:
+			io.Copy(io.Discard, r.Body)
+			w.Header().Set("Location", r.URL.Path)
+			w.WriteHeader(http.StatusAccepted)
 		case http.MethodPut:
 			io.Copy(io.Discard, r.Body)
 			w.WriteHeader(http.StatusCreated)