[Bug]: search iterator v2 fails to fallback because of ambiguous topK parameter

[qixuan0212]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

Use Milvus server version <= 2.5.1 and new pymilvus server from master/2.5.
2025-01-02 10:57:22,838 [WARNING][search_iterator]: The server does not support Search Iterator V2. The search_iterator (v1) is used instead. Please upgrade your Milvus server version to 2.5.2 and later, or use a pymilvus version before 2.5.3 (excluded) to avoid this issue. (milvus_client.py:603) 2025-01-02 10:57:22,881 [ERROR][handler]: RPC error: [search], <ParamError: (code=1, message=ambiguous parameter: topk, in search_param: 200, in search_param.params: 20)>, <Time:{'RPC start': '2025-01-02 10:57:22.880847', 'RPC error': '2025-01-02 10:57:22.881057'}> (decorators.py:140)

Expected Behavior

Fallback to search iterator v1 and be able to run.

Steps/Code To Reproduce behavior

@pytest.mark.tags(CaseLabel.L0)
    def test_milvus_client_search_iterator_default(self, search_params):
        """
        target: test search iterator (high level api) normal case
        method: create connection, collection, insert and search iterator
        expected: search iterator successfully
        """
        client = self._connect(enable_milvus_client_api=True)
        collection_name = cf.gen_unique_str(prefix)
        client_w.using_database(client, "default")
        # 1. create collection
        client_w.create_collection(client, collection_name, default_dim, consistency_level="Bounded")
        collections = client_w.list_collections(client)[0]
        assert collection_name in collections
        client_w.describe_collection(client, collection_name,
                                     check_task=CheckTasks.check_describe_collection_property,
                                     check_items={"collection_name": collection_name,
                                                  "dim": default_dim,
                                                  "consistency_level": 0})
        # 2. insert
        rng = np.random.default_rng(seed=19530)
        rows = [{default_primary_key_field_name: i, default_vector_field_name: list(rng.random((1, default_dim))[0]),
                 default_float_field_name: i * 1.0, default_string_field_name: str(i)} for i in range(default_nb)]
        client_w.insert(client, collection_name, rows)
        client_w.flush(client, collection_name)
        # 3. search iterator
        vectors_to_search = rng.random((1, default_dim))
        insert_ids = [i for i in range(default_nb)]
        client_w.search_interator(client, collection_name, vectors_to_search, search_params=search_params,
                                  check_task=CheckTasks.check_search_iterator,
                                  check_items={"enable_milvus_client_api": True,
                                               "nq": len(vectors_to_search),
                                               "ids": insert_ids,
                                               "limit": default_limit})
        client_w.release_collection(client, collection_name)
        client_w.drop_collection(client, collection_name)

Environment details

- Hardware/Softward conditions (OS, CPU, GPU, Memory): CPU
- Method of installation (Docker, or from source):source
- Milvus version (v0.3.1, or v0.4.0): 2.5.1
- Milvus configuration (Settings you made in `server_config.yaml`):

Anything else?

N/A

[qixuan0212]

/assign @smellthemoon

[PwzXxm]

Fallback logic and V2 does not modify topk.
This may relate to #2537 and search iterator V1

[smellthemoon]

related with #2540