Motivation

• Make Error-related unit tests more focused and actionable by verifying both Display output and current source() semantics for each variant.

Description

• Split the single error_messages_are_readable test into three focused tests: missing_env_var_message_is_readable, invalid_seed_message_is_readable, and io_error_variant_preserves_inner_error.
• Add use std::error::Error as _; and explicit source() assertions to lock in the current error-chaining semantics (non-IO variants and the current IO conversion return no source).
• Rename and adjust assertions to reflect observed behavior of the Io(#[from] std::io::Error) conversion.

Testing

• Ran cargo test -p uselesskey-core error::tests --features std and verified 3 passed; 0 failed for the targeted tests.
• Executed the crate test harness where applicable; other test binaries were filtered by the selection and the run completed successfully.

Codex Task