test: align python client expectations with current API responses (#50414)

## Summary
- update `test_milvus_client_collection_describe` to expect
`properties.max_field_id` in `describe_collection`
- align `search_aggregation` JSON/dynamic field rejection cases with the
current server error messages
- keep the affected Python client regressions covered without asserting
outdated responses

## Test plan
- [x] `python3 -m py_compile
tests/python_client/milvus_client/test_milvus_client_collection.py
tests/python_client/milvus_client/test_milvus_client_search_aggregation.py`
- [ ] full pytest execution was not run locally in this submission

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Signed-off-by: nico <cheng.yuan@zilliz.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: NicoYuan1986

tests/python_client/milvus_client/test_milvus_client_collection.py

@@ -4260,7 +4260,7 @@ def test_milvus_client_collection_describe(self):
             "aliases": [],
             "consistency_level": 0,
             "consistency_level_name": "Strong",
-            "properties": {"timezone": "UTC"},
+            "properties": {"timezone": "UTC", "max_field_id": "102"},
             "num_partitions": 1,
             "enable_dynamic_field": True,
             "enable_namespace": False,

tests/python_client/milvus_client/test_milvus_client_search_aggregation.py

@@ -1154,15 +1154,15 @@ def test_search_aggregation_reject_dynamic_metric_field_with_order(self):
         """
         target: clarify search_aggregation metric/order compatibility with dynamic fields.
         method: aggregate by a static scalar field, compute sum(dynamic_price), and order by that metric alias.
-        expected: request is rejected because dynamic metric sources have no resolved scalar type.
+        expected: request is rejected because JSON / dynamic metric fields are not yet supported with search_aggregation.
         """
         client = self._client()
         collection_name = cf.gen_collection_name_by_testcase_name()
         vectors, vector_field, static_group_field, _, dynamic_metric_field = self._prepare_dynamic_field_collection(
             client, collection_name
         )
 
-        error = {ct.err_code: 999, ct.err_msg: "aggregation operator sum does not support data type None"}
+        error = {ct.err_code: 999, ct.err_msg: "JSON / dynamic fields are not yet supported with search_aggregation"}
         self.search(
             client,
             collection_name,
@@ -1299,7 +1299,10 @@ def test_search_aggregation_reject_json_metric_field_and_sort_path(self):
             client, collection_name
         )
 
-        metric_error = {ct.err_code: 999, ct.err_msg: "aggregation operator sum does not support data type JSON"}
+        metric_error = {
+            ct.err_code: 999,
+            ct.err_msg: "JSON / dynamic fields are not yet supported with search_aggregation",
+        }
         self.search(
             client,
             collection_name,