tests: Add comprehensive edge case and boundary testing for Google GenAI components

[alxkm]

This PR significantly enhances the test coverage for Google GenAI components by adding comprehensive edge case and boundary condition testing across multiple test classes.

Changes

CreateGeminiRequestTests Enhancements

• Thinking Budget Edge Cases: Added tests for null and zero thinking budget values to ensure proper config handling
• Message Boundary Conditions: Added tests for empty message lists and system-message-only scenarios
• Configuration Testing: Added label configuration testing to verify proper metadata handling
• Validation: Ensures proper thinkingConfig presence/absence based on budget values

GoogleGenAiChatOptions Test Improvements

• Zero Value Handling: Added test for zero thinking budget to verify edge case behavior
• Empty Collections: Added test for empty label maps to ensure proper handling of minimal configurations
• Boundary Validation: Ensures options handle edge cases gracefully without exceptions

JsonSchemaConverter Edge Case Coverage

• Empty Input Handling: Added tests for empty JSON objects and empty strings
• Null Safety: Added null input validation to ensure proper exception handling
• Error Scenarios: Validates that appropriate RuntimeExceptions are thrown for invalid inputs

Benefits

• Improved Robustness
• Better Reliability
• Maintenance Benefits
• Regression Prevention

Testing Coverage

The added tests cover:

• Null value handling across all major configuration options
• Empty collection management for lists and maps
• Invalid input scenarios with appropriate exception validation
• Minimal configuration testing to ensure components work with bare-minimum setups
• Boundary condition validation for numeric and string parameters

[ilayaperumalg]

@alxkm Thanks for the PR improving the test coverage. Rebased and merged as 63ba02a