test: add test coverage for MilvusSearchRequest builder

[alxkm]

Description
This PR adds comprehensive test coverage for the MilvusSearchRequest builder class, focusing on various parameter combinations and edge cases.

Changes
Added 7 new test methods to MilvusSearchRequestTest:

• Single parameter tests: Verify builder behavior when setting only one parameter (query, topK, or similarity threshold) while others use defaults
• Empty query handling: Ensures empty strings are handled correctly
• Complex expressions: Tests storage of complex native Milvus expressions
• JSON parameters: Validates complex search parameters JSON handling
• Builder pattern behavior: Verifies that multiple calls to the same setter use the last value

Impact
The existing tests only covered basic scenarios with all parameters set. These new tests ensure the builder works correctly with partial configurations, validates default values are properly applied, and handles edge cases like empty queries and complex expressions.

[ilayaperumalg]

@alxkm Thanks for improving test coverage.

[ilayaperumalg]

Rebased and merged as d016b10