fix: add missing convert_timestamptz in timestamptz default value test

The test_milvus_client_timestamptz_add_field_with_default_value test was
missing a convert_timestamptz() call to normalize expected timestamp values
before comparison. This caused flaky failures when CI environments had
different tzdata versions or when the server returned timestamps in a
slightly different format. All other timestamptz tests already use this
normalization pattern.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: xiaofanluan <xiaofan.luan@zilliz.com>

Author: xiaofan-luan

tests/python_client/milvus_client/test_milvus_client_timestamptz.py

@@ -746,6 +746,7 @@ def test_milvus_client_timestamptz_add_field_with_default_value(self):
         # step 3: query the rows
         for row in rows:
             row[default_timestamp_field_name] = default_timestamp_value
+        rows = cf.convert_timestamptz(rows, default_timestamp_field_name, "UTC")
         self.query(client, collection_name, filter=f"{default_primary_key_field_name} >= 0",
                             check_task=CheckTasks.check_query_results,
                             check_items={exp_res: rows,