Add comprehensive test coverage for Platform.Timestamps library

[konard]

Summary

This PR addresses issue #35 by adding comprehensive test coverage for the Platform.Timestamps library, achieving full coverage of both the Timestamp struct and UniqueTimestampFactory class.

✨ What's Added

• 24 comprehensive tests for Timestamp struct covering:

  • Constructor functionality with ulong ticks
  • Implicit conversions to/from DateTime (UTC handling)
  • Implicit conversions to/from ulong
  • ToString formatting with default format
  • Equality operations (Equals method, == and != operators)
  • GetHashCode implementation consistency
  • Object.Equals overrides with type safety
  • Edge cases (min/max values, round-trip conversions)

• Enhanced UniqueTimestampFactory tests with 9 additional tests covering:

  • Monotonic timestamp generation guarantee
  • Time accuracy validation against current UTC time
  • Uniqueness under rapid consecutive calls (1000 iterations)
  • Concurrent access behavior documentation
  • Long-running stability testing (10,000 iterations)
  • Interface implementation verification
  • Proper UTC timestamp generation

🔧 Code Quality Improvements

• Fixed nullability warning in Timestamp.Equals(object?) method to comply with nullable reference types
• All tests use AAA pattern (Arrange, Act, Assert) for clarity
• Comprehensive edge case testing including boundary values
• Thread-safety behavior documentation for concurrent usage

📊 Test Results

• Total tests: 34
• All tests passing: ✅ 34/34
• Build status: ✅ No warnings or errors
• Coverage: Complete coverage of all public APIs

🧪 Test Categories

1. Unit Tests: Core functionality of both classes
2. Integration Tests: DateTime conversion accuracy
3. Performance Tests: High-volume timestamp generation
4. Concurrency Tests: Multi-threaded access patterns
5. Edge Case Tests: Boundary values and error conditions

Test Plan

✅ All existing functionality preserved
✅ New comprehensive test suite passing
✅ Build completed without warnings
✅ Nullability compliance achieved
✅ Performance characteristics validated

Breaking Changes

❌ None - This is purely additive test coverage with one minor nullability fix that doesn't affect public API.

🤖 Generated with Claude Code

---

Resolves #35