Backend-level test coverage for remaining 10 backends

[trmxvibs]

tests/backends/test_apt_backend.py is the only backend with dedicated
unit tests right now. The same mocking pattern should be applied to the
other 10 backends (pacman, dnf, apk, brew, zypper, xbps, nix, eopkg, freebsd, pkg/Termux) to catch regressions early.

[kaminimangal]

@trmxvibs Can you assign this to me? I already added the Windows backend and the package mapping (#3, #5), so I'm familiar with the test pattern in test_apt_backend.py. Plan is to write the same style of mocked tests for the other 10 backends (pacman, dnf, apk, brew, zypper, xbps, nix, eopkg, freebsd, pkg).

[trmxvibs]

Partly addressed in v0.2.0. tests/backends/test_all_backends.py now runs
a shared contract suite against every registered backend, checking that:

• the registry name matches the class and the declared executable
• install and remove pass through every package argument
• no non-interactive flag is added unless -y was given
• search never requires root and forwards the query
• backends without a native prompt supply a pkgwrap confirmation on removal
• dry_run is forwarded

New backends are picked up automatically, so this class of gap can't
reappear.

What's still missing is focused per-backend tests that pin the exact
command shape — currently apt, winget, choco, emerge, openbsd,
port and yum have those. The remaining 10 would benefit from the same
treatment, especially the flag-heavy ones (pacman, xbps, nix).

Still a good first issue: pick one backend, copy test_apt_backend.py,
assert the exact command lists. Please say in your PR which backend you
took so two people don't do the same one.