test: cover Result residual conversion for Exit

[wenxiaojie1]

Fixes #176.\n\nAdds a regression test that uses ? on Result<(), String> in a function returning the derived Exit<()>, then verifies the error is converted to Exit::OtherError. The test-local From implementation supplies the residual conversion required by the derive.\n\nValidation:\n- git diff --check\n- Confirmed the test exercises the Result ? path\n\nThe local Windows environment has no Cargo/Rust toolchain, so repository CI is the compilation and test gate.

[sourcery-ai]

Hey - I've left some high level feedback:

• The blanket impl<T: Termination> From<String> for Exit<T> in the test may prevent adding a more general From implementation for Exit in the main crate later due to coherence rules; consider narrowing the impl (e.g., to a test-only newtype) so it cannot conflict with future library code.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The blanket `impl<T: Termination> From<String> for Exit<T>` in the test may prevent adding a more general `From` implementation for `Exit` in the main crate later due to coherence rules; consider narrowing the impl (e.g., to a test-only newtype) so it cannot conflict with future library code.

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[wenxiaojie1]

Addressed the review: the test conversion is now limited to the actual residual type, Exit<!>, rather than applying to every Exit. This keeps the test focused on the derived Result residual path.