test: add struct array element hybrid correctness coverage (#50146)

## What this PR does

Adds Python client correctness coverage for struct array element-level +
normal field hybrid search.

The new case:

- inserts more than 3000 rows with multi-element struct arrays
- computes brute-force ground truth from inserted data
- verifies raw element-level search results before collapse
- verifies normal vector search results
- verifies hybrid RRF results after element-level row collapse
- covers both FLAT and HNSW indexes

## Verification

Ran against a master standalone instance: `master-20260528-d44e5bcdc4`.

```bash
python -m pytest milvus_client/test_milvus_client_struct_array_element_search.py::TestMilvusClientStructArrayElementHybridSearch::test_hybrid_element_filter_collapse_correctness --host 10.100.36.224 --port 19530 --token root:Milvus --tb=short -q
```

Repeated 3 times:

```text
run 1/3: 2 passed in 23.44s
run 2/3: 2 passed in 26.81s
run 3/3: 2 passed in 21.81s
```

Signed-off-by: zhuwenxing <wenxing.zhu@zilliz.com>