test: make the remaining uv download tests pass without uv installed

[espressolee]

Fixes #1045. Same root cause as #1046 — three call sites #1137 did not cover.

Reproduction on main @7519a98

$ python -m pytest tests/test_virtualenv.py -k download_python -q
33 passed

$ env -i PATH="$PWD/.venv/bin:/usr/bin:/bin" python -m pytest tests/test_virtualenv.py -k download_python -q
FAILED test_download_python_auto_missing_interpreter[uv]
FAILED test_download_python_always_preexisting_interpreter[uv]
FAILED test_download_python_range_installs_floor[always-uv]
FAILED test_download_python_range_installs_floor[auto-uv]
4 failed, 29 passed

Cause

nox/virtualenv.py:941-949:

if self.venv_backend == "uv":
    has_uv, _, uv_ver = _uv_state()
    if (
        has_uv
        and version.Version("0.4.16") <= uv_ver
        and uv_install_python(cleaned_interpreter)
    ):

Without uv on the host, has_uv is False and the mocked uv_install_python
is never reached, so assert_called_once_with(...) cannot hold.

This issue surfaces as InterpreterNotFound rather than #1046's assertion
error only because test_download_python_failed_install already wraps the call
in pytest.raises. Same short-circuit, different symptom.

Change

The guard #1137 added, applied to the three remaining tests that assert
uv_install_mock.assert_called_once_with(...). No production code touched, so
coverage is unaffected.

Verification

pytest tests/test_virtualenv.py (whole file):

	without uv	with uv
before	4 failed, 156 passed	7 failed, 187 passed
after	0 failed, 160 passed	7 failed, 187 passed

The 7 with-uv failures are conda tests. They are byte-identical before and
after — I diffed the two FAILED lists — and I did not investigate their
cause, since this change does not touch that path.

Removing HAS_UV from one of the three new guards turns exactly that one test
red again, so the guard is load-bearing rather than decorative.

ruff check and ruff format --check pass on the changed file.

[henryiii]

Thanks! I swapped the Claude coauthor for Assisted-by when squashing.