test: Add comprehensive test coverage for Neo4jVectorFilterExpressionConverter

[alxkm]

Overview
This PR adds new test cases to significantly improve the robustness of our Neo4j filter expression converter. These tests cover edge cases and complex scenarios that users encounter in real-world applications.

New Test Cases
Basic Edge Cases:

• testEmptyList() - IN operations with empty lists
• testSingleItemList() - IN operations with single items
• testNullValue() - Equality checks with null values
• testZeroValue() - Edge case for zero values
• testEmptyStringValue() - Not-equal comparisons with empty strings

Data Type Handling:

• testNumericStringValue() - Numeric values stored as strings
• testMixedDataTypes() - Complex expressions mixing booleans, decimals, strings, and integers
• testNegativeNumbers() - Negative numeric value handling

Another Scenarios:

• testNestedJsonPath() - Nested field paths like entity.profile.name
• testArrayIndexAccess() - Direct array element access like tags[0]
• testComplexNestedGroups() - Multi-level boolean logic with nested AND/OR groupings
• testNinWithMixedTypes() - NOT IN operations across different data types

Thank you for taking time to contribute this pull request!
You might have already read the contributor guide, but as a reminder, please make sure to:

• Add a Signed-off-by line to each commit (git commit -s) per the DCO
• Rebase your changes on the latest main branch and squash your commits
• Add/Update unit tests as needed
• Run a build and make sure all tests pass prior to submission

For more details, please check the contributor guide.
Thank you upfront!