ci : cap test-backend-ops parallel jobs at 2 and add a 3600s timeout (#28833)

- Clamp the -j parallelism to min(nproc, 2) so a single-core runner
  uses -j 1 and multi-core runners use at most -j 2, instead of
  unconditionally using $(nproc).
- Add a 3600s timeout to both test-backend-ops runs (the high-perf CPU
  path and the default path) so a hung test cannot stall CI indefinitely.
- Note a TODO to reduce the timeout to 1800s in the future.

Assisted-by: pi:llama.cpp/Qwen3.8-27B

ci/run.sh

@@ -775,7 +775,11 @@ function gg_run_test_backend_ops {
 
     set -e
 
-    local args_extra="-j $(nproc)"
+    local n_jobs=$(nproc)
+    if [ "${n_jobs}" -gt 2 ]; then
+        n_jobs=2
+    fi
+    local args_extra="-j ${n_jobs}"
 
     # TODO: fix multi-threaded for ROCm
     #       https://github.com/ggml-org/llama.cpp/actions/runs/34576278519/job/103297889044?pr=28740#step:3:4865
@@ -789,10 +793,11 @@ function gg_run_test_backend_ops {
         args_extra=""
     fi
 
+    # TODO: reduce the test-backend-ops timeout to 1800s
     if [ ! -z ${GG_BUILD_HIGH_PERF} ]; then
-        (time ./bin/test-backend-ops ${args_extra} -b CPU) 2>&1 | tee -a $OUT/${ci}-test-backend-ops.log
+        (time timeout 3600 ./bin/test-backend-ops ${args_extra} -b CPU) 2>&1 | tee -a $OUT/${ci}-test-backend-ops.log
     else
-        (time ./bin/test-backend-ops ${args_extra}       ) 2>&1 | tee -a $OUT/${ci}-test-backend-ops.log
+        (time timeout 3600 ./bin/test-backend-ops ${args_extra}       ) 2>&1 | tee -a $OUT/${ci}-test-backend-ops.log
     fi
 
     set +e