Test: Add comprehensive test suites for server, shell, E2E, and interop

[hahwul]

Summary

Several major modules lack test coverage:

1. REST API server (cmd/server.rs) - No endpoint tests
2. Interactive shell (cmd/shell/) - No command execution tests
3. E2E scenarios - No full workflow tests (encode → crack → verify)
4. Interoperability - No cross-library JWT/JWE compatibility tests

Acceptance Criteria

• Server: Test all API endpoints with mock requests
• Shell: Test command parsing and execution
• E2E: At least 3 full workflow scenarios
• Interop: Validate tokens against at least one other JWT library