Fix PR review comments for cache verification and docs wording

Co-authored-by: henryiii <4616906+henryiii@users.noreply.github.com>

unit_test/python_build_standalone_test.py

@@ -0,0 +1,63 @@
+from __future__ import annotations
+
+import hashlib
+
+from cibuildwheel.util import python_build_standalone
+
+TYPE_CHECKING = False
+if TYPE_CHECKING:
+    from pathlib import Path
+
+    import pytest
+
+
+def test_download_or_get_from_cache_uses_valid_cached_file(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    cached_file = tmp_path / "python-build-standalone.tar.gz"
+    cached_bytes = b"cached archive"
+    cached_file.write_bytes(cached_bytes)
+    cached_sha256 = hashlib.sha256(cached_bytes).hexdigest()
+
+    was_downloaded = False
+
+    def fake_download(url: str, dest: Path, *, sha256: str | None = None) -> None:
+        nonlocal was_downloaded
+        was_downloaded = True
+
+    monkeypatch.setattr(python_build_standalone, "download", fake_download)
+
+    archive_path = python_build_standalone._download_or_get_from_cache(
+        asset_url="https://example.com/python-build-standalone.tar.gz",
+        asset_filename=cached_file.name,
+        cache_dir=tmp_path,
+        sha256=cached_sha256,
+    )
+
+    assert archive_path == cached_file
+    assert not was_downloaded
+
+
+def test_download_or_get_from_cache_redownloads_invalid_cached_file(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    cached_file = tmp_path / "python-build-standalone.tar.gz"
+    cached_file.write_bytes(b"bad cache")
+    expected_bytes = b"good archive"
+    expected_sha256 = hashlib.sha256(expected_bytes).hexdigest()
+
+    def fake_download(url: str, dest: Path, *, sha256: str | None = None) -> None:
+        assert sha256 == expected_sha256
+        dest.write_bytes(expected_bytes)
+
+    monkeypatch.setattr(python_build_standalone, "download", fake_download)
+
+    archive_path = python_build_standalone._download_or_get_from_cache(
+        asset_url="https://example.com/python-build-standalone.tar.gz",
+        asset_filename=cached_file.name,
+        cache_dir=tmp_path,
+        sha256=expected_sha256,
+    )
+
+    assert archive_path == cached_file
+    assert cached_file.read_bytes() == expected_bytes