Repair macOS cmd-mox failure propagation in generate-coverage tests

[coderabbitai]

Summary

Repair .github/actions/generate-coverage/tests/test_scripts.py::test_run_rust_with_cucumber_cobertura_merge_failure on macOS.

Evidence

PR #361 currently fails python-tests (macos-15). The test expects the merge-cobertura stub exit code 3. The subprocess returns 1 after cmd-mox reports malformed JSON and the IPC server reports BrokenPipeError.

Scope

• Reproduce the failure on macOS Python 3.13.
• Identify whether the fault is in the cmd-mox stub setup, command invocation, or cmd-mox compatibility.
• Restore propagation of the merge-tool exit code.
• Add regression coverage that detects malformed IPC payloads and preserves the expected exit code.

Acceptance criteria

• The named test passes on macOS.
• The expected exit code 3 reaches the caller.
• Linux and Windows test behaviour remains unchanged.

Backlinks

• PR: Add install-whitaker shared action #361
• Requested by: @leynos