test: Add additional test coverage for OpenSearchAiSearchFilterExpressionConverter

[alxkm]

Added additional test cases for OpenSearch filter expression converter:

• testEmptyList() - Handles IN operations with empty lists gracefully
• testSingleItemList() - Validates IN operations with single-item lists
• testNullValue() - Tests equality checks against null values
• testNestedJsonPath() - Supports nested field paths like entity.profile.name
• testNumericStringValue() - Properly handles numeric values as strings
• testZeroValue() - Edge case testing for zero values
• testComplexNestedGroups() - Complex boolean logic with multiple AND/OR groupings
• testMixedDataTypes() - Real-world scenarios mixing booleans, numbers, and strings
• testNinWithMixedTypes() - NOT IN operations with various data types
• testEmptyStringValue() - Not-equal comparisons with empty strings
• testArrayIndexAccess() - Direct array element access like tags[0]