- Rename authorization path constants to avoid conflict with existing authType enum
- Replace nested if/else with clean switch statement in authorizeWithIAM()
- Add determineIAMAuthPath() helper for clearer intent and testability
- Optimize key counting in auth_signature_v4.go: remove unnecessary slice allocation
- Fix timing assertion in session_claims_test.go: use WithinDuration for symmetric tolerance
These changes improve code readability, maintainability, and performance while
maintaining full backward compatibility and test coverage.