[CI] Refator multi-node CI (#3487)

### What this PR does / why we need it?
Refactor the multi-machine CI use case. The purpose of this PR is to
increase the ease of adding multi-machine CI use cases, allowing
developers to add multi-machine cluster model testing use cases
(including PD separation) by simply adding a new YAML configuration
file.
### Does this PR introduce _any_ user-facing change?

### How was this patch tested?

- vLLM version: v0.11.0rc3
- vLLM main: https://github.com/vllm-project/vllm/commit/v0.11.0

---------

Signed-off-by: wangli <wangli858794774@gmail.com>

.github/workflows/multi_node_test.yaml

@@ -102,6 +102,15 @@ jobs:
             wait $LOG_PID || true
             kill $MONITOR_PID || true
 
+        - name: Generate summary
+          if: always()
+          run: |
+            if [ -f "/root/.cache/test_summary.md" ]; then
+              cat /root/.cache/test_summary.md >> "$GITHUB_STEP_SUMMARY"
+            else
+              echo "No summary file found." >> "$GITHUB_STEP_SUMMARY"
+            fi
+
         - name: Post process
           if: always()
           run: |