Adds comprehensive test coverage for Document and DocumentBuilder classes focusing on:

Document Tests:

• Metadata mode handling
• Complex metadata structures and immutability
• Document mutation and equality edge cases
• Score validation
• Text content edge cases (whitespace, unicode)
• Hash code consistency

DocumentBuilder Tests:

• Media vs text content validation and exclusivity
• Metadata key validation with special characters
• Builder state isolation between multiple builds
• Method chaining verification
• Content overwriting scenarios
• Text formatting preservation (newlines, tabs)
• Metadata accumulation vs replacement behavior

These tests improve robustness by covering previously untested edge cases, validation scenarios, and ensuring proper isolation between builder instances.

Test Coverage Improvements:

• Added 21 new test methods across both test classes
• Enhanced validation of error conditions
• Improved coverage of builder pattern edge cases
• Better testing of metadata handling complexity