chore: improve rustlings maintenance path

[vip892766gma]

Summary:

• Add unit tests covering edge cases (empty input, malformed TOML, missing fields) and tighten explicit error branches in cargo_toml parsing helpers; add small assertion tests for terminal helper functions in term.rs.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.