fix(ci): fail closed when workspace matrix discovery produces empty list

The set-matrix step wrote the npm workspace query result directly to
$GITHUB_OUTPUT. If discovery ever produced [], the matrix would expand
to zero check jobs, leaving the reusable workflow green without running
any JS/TS checks.

Now the step validates the result is a non-empty array before emitting
it, and exits 1 with a GitHub annotation if it's empty or jq failed.

.github/workflows/js-tests.yml

@@ -21,7 +21,12 @@ jobs:
           command: npm ci --ignore-scripts
       - id: set-matrix
         run: |
-          echo "packages=$(npm query .workspace | jq -c '[.[].location]')" >> "$GITHUB_OUTPUT"
+          PACKAGES=$(npm query .workspace | jq -c '[.[].location]')
+          if [ "$PACKAGES" = "[]" ] || [ -z "$PACKAGES" ]; then
+            echo "::error::Workspace discovery produced an empty package list — refusing to emit a zero-length matrix (would skip all JS/TS checks silently)."
+            exit 1
+          fi
+          echo "packages=$PACKAGES" >> "$GITHUB_OUTPUT"
 
   check:
     name: Typecheck & Test