Add comprehensive test utility and error handling imports to encryption test files:
- Added std::error::Error and std::io for error handling
- Expanded pdftract_core::diagnostics imports to include Diagnostic, DiagnosticsCollector, and ObjRef
- Ensured both test_encryption_errors.rs and test_encryption_unsupported.rs have consistent imports
All imports verified against actual crate structure and compile successfully.
Closes bf-1h5og. Verification: notes/bf-1h5og.md.
