test: Add comprehensive Document class validation and functionality tests

[alxkm]

Adds essential test coverage for the Document class to improve code quality and prevent regressions.

Tests Added:

• Input validation (null/empty ID, metadata validation)
• Builder pattern validation and custom ID generator support
• Content formatter functionality and error handling
• Document mutation ensuring immutability
• Equals/hashCode contract verification
• Text document behavior validation
• Metadata immutability guarantees

Benefits:

• Prevents runtime errors through comprehensive validation testing
• Ensures API contracts are maintained during refactoring
• Improves confidence in Document class reliability
• Covers edge cases and error conditions

Coverage: 19 new test methods covering validation, builder patterns, mutation, equality, and core functionality.