Recreate venv if python interpreter is missing (#216)

This fixes a bug where a cached venv might become broken due to python being upgraded and
tox was unable to recreate it:
```
error: Failed to inspect Python interpreter from active virtual environment at `.tox/py314/bin/python3`
  Caused by: Broken symlink at `.tox/py314/bin/python3`, was the underlying Python interpreter removed?

hint: Consider recreating the environment (e.g., with `uv venv`)
py314: exit 2 (0.01 seconds) /home/runner/work/ansible-lint/ansible-lint> .tox/.tox/bin/uv pip install --reinstall --no-deps ansible-lint@/home/runner/work/ansible-lint/ansible-lint/.tox/.tmp/package/2/ansible_lint-25.6.2.dev4-0.editable-py3-none-any.whl pid=2959
```

See: https://github.com/ansible/ansible-lint/actions/runs/15822605275/job/44605268272?pr=4637

Author: ssbarnea

src/tox_uv/_installer.py

@@ -87,6 +87,12 @@ def post_process_install_command(self, cmd: Command) -> Command:
         return cmd
 
     def install(self, arguments: Any, section: str, of_type: str) -> None:  # noqa: ANN401
+        # can happen if the original python was upgraded to a newer version and
+        # the symlinks become orphan.
+        if not self._env.env_python().resolve().is_file():
+            msg = "existing venv is broken"
+            raise Recreate(msg)
+
         if isinstance(arguments, PythonDeps):
             self._install_requirement_file(arguments, section, of_type)
         elif isinstance(arguments, Sequence):  # pragma: no branch

tests/test_tox_uv_installer.py

@@ -123,3 +123,29 @@ def test_uv_install_with_resolution_strategy_and_pip_pre(tox_project: ToxProject
         "tomli>=2.0.1",
         "-v",
     ]
+
+
+def test_uv_install_broken_venv(tox_project: ToxProjectCreator) -> None:
+    """Tests ability to detect that a venv a with broken symlink to python interpreter is recreated."""
+    project = tox_project({
+        "tox.ini": """
+    [testenv]
+    skip_install = true
+    install = false
+    commands = python3 --version
+    """
+    })
+    result = project.run("run", "-v")
+    result.assert_success()
+    assert "recreate env because existing venv is broken" not in result.out
+    # break the environment
+    bin_dir = project.path / ".tox" / "py" / "bin"
+    bin_dir.mkdir(parents=True, exist_ok=True)
+    for filename in ("python", "python3"):
+        path = bin_dir / filename
+        path.unlink()
+        path.symlink_to("/broken-location")
+    # run again and ensure we did run the repair bits
+    result = project.run("run", "-v")
+    result.assert_success()
+    assert "recreate env because existing venv is broken" in result.out