ci: 👷 Add CI job to check rust tests results if they run (#16)

This PR adds a new job at the end of the `DataHave Operator Rust Tests`
(`rust-tests.yml`) that checks:
- If the tests were skipped (no changes in the `operator/` directory),
it succeeds.
- If the tests run and succeeded, it succeeds.
- If the tests run and failed, it fails.

This is useful to set this job as required in our merging ruleset, and
account for cases where the tests are skipped.

.github/workflows/rust-tests.yml

@@ -107,3 +107,25 @@ jobs:
           ~/.cargo/bin/cargo-nextest nextest run \
             --archive-file ../nextest-archive.tar.zst \
             --partition count:${{ matrix.partition }}/2
+
+  tests-result-checker:
+    name: Check tests were successful
+    needs: [setup, all-rust-tests]
+    if: always()
+    runs-on: ubuntu-latest
+    steps:
+      - name: Validate test results
+        run: |
+          echo "node_changed: ${{ needs.setup.outputs.node_changed }}"
+          echo "matrix result: ${{ needs.all-rust-tests.result }}"
+
+          if [ "${{ needs.setup.outputs.node_changed }}" == "true" ]; then
+            if [ "${{ needs.all-rust-tests.result }}" != "success" ]; then
+              echo "Rust tests failed or were cancelled"
+              exit 1
+            else
+              echo "Rust tests passed"
+            fi
+          else
+            echo "No relevant changes — skipping rust tests"
+          fi