test: Add unit tests for config parsing edge cases

[josedab]

Description

The configuration parser in src/config.rs could benefit from additional edge case tests.

Task

• Add tests for: empty config file, invalid TOML, missing required fields, out-of-range numeric values
• Follow the existing test pattern in the file
• Run cargo test to verify

Helpful Context

• Look at existing tests at the bottom of src/config.rs
• Use tempfile crate for temporary config files