Enhance test coverage for tool execution related classes with additional edge cases and validation scenarios:

• DefaultToolCallingChatOptions: Added tests for default constructor initialization, empty collections handling, and null value acceptance for internal tool execution flag
• DefaultToolExecutionEligibilityPredicate: Added tests for multiple generations scenarios, empty tool calls lists, and multiple tool calls handling
• DefaultToolExecutionResult: Added comprehensive tests for builder behavior, conversation history validation, state mutations, and object contracts (equals/hashCode)

These tests improve robustness by covering previously untested edge cases and ensuring proper validation behavior across the tool execution framework.