Add UpsertMultiple diagnostic step to example

UpsertMultiple returns 400 Bad Request on custom tables while single
PATCH upsert works correctly. This is a pre-existing issue in the
_upsert_multiple implementation (PR #106) that needs investigation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: tpellissier

examples/advanced/alternate_keys_upsert.py

@@ -158,6 +158,30 @@ def main():
     )
     print("   Upserted EXT-002 (single)")
 
+    print("\n4c. Upsert multiple records (UpsertMultiple bulk)...")
+    try:
+        client.records.upsert(
+            TABLE_NAME,
+            [
+                UpsertItem(
+                    alternate_key={KEY_COLUMN.lower(): "EXT-003"},
+                    record={"new_productname": "Widget C", "new_price": 29.99},
+                ),
+                UpsertItem(
+                    alternate_key={KEY_COLUMN.lower(): "EXT-004"},
+                    record={"new_productname": "Widget D", "new_price": 39.99},
+                ),
+            ],
+        )
+        print("   Upserted EXT-003, EXT-004 (bulk)")
+    except Exception as ex:  # noqa: BLE001
+        print(f"   UpsertMultiple FAILED: {ex}")
+        if hasattr(ex, "status_code"):
+            print(f"   Status: {ex.status_code}")
+        if hasattr(ex, "service_error_code"):
+            print(f"   Service error code: {ex.service_error_code}")
+        print("   (single upsert worked, bulk did not - known issue under investigation)")
+
     # ------------------------------------------------------------------
     # Step 5: Upsert again (updates existing via single PATCH)
     # ------------------------------------------------------------------