Add comprehensive unit tests

[genro]

Context

Core logic needs unit tests to ensure correctness and prevent regressions. Priority is TokenManager, providers, and storage.

Goal

Achieve >80% code coverage with fast unit tests.

Implementation

• Create tests/test_opaque_provider.py
  • Test generate() creates valid tokens
  • Test validate() accepts valid tokens
  • Test validate() rejects invalid/expired tokens
  • Test refresh() creates new token pair
  • Test revoke() removes tokens
• Create tests/test_token_manager.py
  • Test manager with configured provider
  • Test configuration API
  • Test error handling
• Create tests/test_memory_storage.py
  • Test set/get/delete operations
  • Test datetime serialization
  • Test clear() method
• Create tests/conftest.py with fixtures
• Run pytest with coverage

Acceptance Criteria

• All tests pass
• Coverage >80%
• Tests run in <5 seconds
• No external dependencies (no Redis, DB)
• Clear test names (test_validate_returns_none_for_expired_token)

Dependencies

• Requires: Refactor to provider pattern for multi-token-type support #1 (provider pattern)
• Can run parallel to: Add declarative configuration API #2, Add FastAPI integration with dependencies #3

Notes

• Use pytest
• Mock time for expiration tests
• Test edge cases: expired, invalid, malformed tokens
• Test concurrency safety (if needed)