Commented out unused internal module imports in encryption error tests
with comprehensive documentation explaining the subprocess testing
approach. Verified all modules exist and compile successfully.
Verified imports:
- pdftract_cli::password (crates/pdftract-cli/src/password.rs)
- pdftract_core::diagnostics types (crates/pdftract-core/src/diagnostics.rs)
Tests use std::process::Command to spawn CLI binary, not direct
module calls. Imports retained as comments for future reference.
Closes bf-31re9. Verification: notes/bf-31re9.md
