The tools crate already covered several higher-level commands, but the
public dispatch surface still lacked direct tests for shell and file
operations plus several error-path behaviors. This change expands the
existing lib.rs unit suite to cover the requested tools through
`execute_tool`, adds deterministic temp-path helpers, and hardens
assertions around invalid inputs and tricky offset/background behavior.
Constraint: No new dependencies; coverage had to stay within the existing crate test structure
Rejected: Split coverage into new integration tests under tests/ | would require broader visibility churn for little gain
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep future tool-coverage additions on the public dispatch surface unless a lower-level helper contract specifically needs direct testing
Tested: cargo fmt --all; cargo clippy -p tools --all-targets --all-features -- -D warnings; cargo test -p tools
Not-tested: Cross-platform shell/runtime differences beyond the current Linux-like CI environment
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