test: add e2e cases for nullable vector backup and restore

[huanghaoyuanhhy]

Summary

Add end-to-end coverage for nullable vector fields (Milvus 2.6.18 / 3.0.0 feature) through the backup/restore round-trip. Existing nullable tests only cover scalar fields (FLOAT, VARCHAR, JSON, ARRAY) — there is no coverage that the nullable=true flag and per-row NULL semantics survive backup + restore for vector fields.

Changes

• Add tests/testcases/test_restore_nullable_vector.py with four test cases:
  • test_restore_nullable_vector_round_trip parameterized over the six vector dtypes in scope (FLOAT, FLOAT16, BFLOAT16, INT8, BINARY, SPARSE_FLOAT): asserts the restored field keeps nullable=true and that NULL / non-NULL rows are preserved per primary key.
  • test_restore_nullable_vector_search_skips_null: builds an index on the restored collection and confirms search hits are at most the non-NULL row count and never land on a NULL row.
  • test_restore_add_nullable_vector_field: exercises add_collection_field with a nullable vector field, then checks the field round-trips and pre-add rows come back as NULL.
  • test_restore_nullable_vector_with_skip_create_collection: negative case asserting that restoring NULL vector rows into a pre-created target with nullable=false fails fast (L2; expected to harden further once the restore path adds a pre-flight nullable schema check).

The tests bypass compare_collections and inspect rows directly via MilvusClient.query, because compare_collections does a strict src_res[i] == dist_res[i] which is ambiguous for numpy ndarray vector values and may not handle nullable vectors cleanly.

/kind improvement

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 42.50%. Comparing base (2309945) to head (43cfa30).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   42.50%   42.50%           
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  Files         136      136           
  Lines       12181    12181           
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  Hits         5178     5178           
  Misses       6624     6624           
  Partials      379      379           

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