fix: replace assert with proper exception in python_build_standalone (#2859)

Assert statements are skipped under python -O, making them unsuitable
for runtime validation. Replace assert with an explicit check that
raises PythonBuildStandaloneError if the target directory already exists.

Assisted-by: OpenCode:glm-5

cibuildwheel/util/python_build_standalone.py

@@ -175,7 +175,9 @@ def create_python_build_standalone_environment(
     )
 
     python_base_dir = temp_dir / "pbs"
-    assert not python_base_dir.exists()
+    if python_base_dir.exists():
+        msg = f"python-build-standalone directory already exists: {python_base_dir}"
+        raise PythonBuildStandaloneError(msg)
     extract_tar(archive_path, python_base_dir)
 
     return _find_python_executable(python_base_dir)