fix(windows): restore MSVC lint for CLI and MXC

[feloy]

Summary

Restore Windows MSVC lint by handling Unix-only SSH cancellation signatures in Clippy and updating the MXC mocked-backend constructor call. These changes keep runtime behavior unchanged.

Related Issue

Closes #4190
Closes #4191

Changes

• Scope three Clippy allowances to non-Unix builds where signal handling is compiled out.
• Pass the default gateway name when constructing the mocked MXC backend in tests.

Testing

• Checks appropriate to the affected code and behavior pass:
  • mise run --skip-tools windows:lint:x64 passed on a local Windows machine (reported by the PR author).
  • cargo fmt --all -- --check passed on macOS.
  • cargo clippy -p openshell-cli --all-targets -- -D warnings passed on macOS.
  • Repository commit hooks passed for both commits, including workspace Rust lint.
• Unit tests added/updated (not applicable: these are lint and test-target compilation fixes).
• E2E tests added/updated (not applicable).
• Windows ARM64 lint and tests have not been run.

Checklist

• Follows Conventional Commits.
• Commits are signed off (DCO).
• Architecture docs updated (not applicable; no architecture or user-facing behavior changed).

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[feloy]

Needs the test:windows label