What this PR does / why we need it:

Milvus search params were hard-coded to {"nprobe": 10} and index params to {"nlist": nlist}.
index_type: AUTOINDEX (the only option on Zilliz Cloud / managed Milvus) failed with
only metric type can be passed when use AutoIndex, for two reasons: nlist was always sent, and
collection_name was passed to IndexParams.add_index, which treats unknown kwargs as index params.

• New index_params and search_params dicts are passed through to Milvus.
• When unset, the old defaults are kept, except that AUTOINDEX gets no params.
• collection_name is no longer passed to add_index.
• AUTOINDEX can be tuned with search_params: {level: N}.

online_store:
  type: milvus
  index_type: "AUTOINDEX"
  search_params:
    level: 2

Which issue(s) this PR fixes:

N/A

Checks

• I've made sure the tests are passing.
• My commits are signed off (git commit -s)
• My PR title follows conventional commits format

Testing Strategy

• Unit tests

• Integration tests

• Manual tests

• Testing is not required for this change

• Unit (mocked client): default params unchanged for IVF_FLAT; AUTOINDEX sends only the metric;
  HNSW index_params/search_params pass through.

• Unit (Milvus Lite): AUTOINDEX + level search round trip.

• Server: AUTOINDEX collection is created and searchable with level: 2.

Unit tests run on Milvus Lite 3.2.1 (pymilvus 3.0.2). Server tests in sdk/python/tests/integration/online_store/test_milvus_remote.py are marked integration and skip unless ZILLIZ_URI and ZILLIZ_TOKEN are set; they passed against a local Milvus 2.6.0 server and against Zilliz Cloud. The existing Milvus unit and universal integration tests pass unchanged.

Misc

Part of a series of Milvus online store improvements for Zilliz Cloud and production Milvus, following #6882 and #6895.