Add branch-focused tests for pet-homebrew locator #393

Summary

Add focused unit tests for pet-homebrew to cover the main locator and helper branches called out in #389, without expanding the existing broad coverage PR.

Scope

• Cover locator metadata and environment identification for Homebrew Python installs
• Cover non-Homebrew rejection paths
• Cover symlink and prefix derivation helpers where feasible with temp fixtures
• Keep this as a small, crate-scoped follow-up to PR Improve locator and reporter test coverage #392

Validation

• cargo test -p pet-homebrew --lib
• cargo fmt --all
• cargo clippy --all -- -D warnings

Part of #389.