Enhances test coverage with additional edge cases and scenarios:

KeywordMetadataEnricher:

• Empty documents list handling
• Single document processing
• Existing metadata preservation
• Empty/whitespace response handling
• Keyword overwriting behavior
• Builder pattern with mixed configurations
• Special characters in content

ToolRuntimeHints:

• Custom ClassLoader support
• Multiple registration calls
• Instance creation validation

Usage:

• Zero token edge cases for prompt/completion tokens
• Total token calculation with various inputs

These tests improve reliability by covering previously untested paths and edge cases.