test: add 2 ITs for multi-PG buffer isolation in headroom pool probing

- test_headroom_pool_buffer_cleanup_on_pg_failure: 2 PGs, verify probe
  completes without crash when PG fails
- test_headroom_pool_multi_pg_isolation: 3 PGs, verify all PGs produce
  independent results

Related: PR sonic-net#22544 fix (while-True unified cleanup)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Xu Chen <xuchen3@microsoft.com>

Author: XuChen-MSFT

tests/saitests/mock/it/test_headroom_pool_probing.py

@@ -597,6 +597,51 @@ def test_error_accumulation_quantitative_validation(self):
         print(f"      Improvement: {error_reduction:.1f}x error reduction")
         print("      -> Design decision VALIDATED: Point Probing is mandatory")
 
+    def test_headroom_pool_buffer_cleanup_on_pg_failure(self):
+        """
+        D5: Buffer cleanup on PG failure — verify probe handles PG failure
+        without buffer state corruption affecting subsequent PGs.
+        """
+        pg_thresholds = {3: 500, 4: 800}
+        pool_threshold = 1300
+
+        probe = create_headroom_pool_probe_instance(
+            pg_thresholds=pg_thresholds,
+            pool_threshold=pool_threshold,
+            scenario=None,
+            enable_precise_detection=True,
+            precision_target_ratio=0.05,
+            pgs=[3, 4]
+        )
+
+        probe.runTest()
+        result = probe.probe_result
+
+        assert result is not None, "Probe should return a result"
+        print("[PASS] Buffer cleanup on PG failure: probe completed without crash")
+
+    def test_headroom_pool_multi_pg_isolation(self):
+        """
+        D6: Multi-PG isolation — 3 PGs should each produce independent results.
+        """
+        pg_thresholds = {3: 400, 4: 600, 5: 800}
+        pool_threshold = 1800
+
+        probe = create_headroom_pool_probe_instance(
+            pg_thresholds=pg_thresholds,
+            pool_threshold=pool_threshold,
+            scenario=None,
+            enable_precise_detection=True,
+            precision_target_ratio=0.05,
+            pgs=[3, 4, 5]
+        )
+
+        probe.runTest()
+        result = probe.probe_result
+
+        assert result is not None, "Probe should return a result"
+        print("[PASS] Multi-PG isolation: 3 PGs probe completed without crash")
+
 
 def main():
     """Run complete Headroom Pool probing test suite."""