test: make the mock harness explicitly mock-only and fail fast with real HA

tests/test_harness.py

@@ -0,0 +1,46 @@
+"""Contract test for the mock-only homeassistant harness in conftest.py."""
+
+from __future__ import annotations
+
+import importlib.metadata
+import sys
+from unittest.mock import MagicMock
+
+import pytest
+
+import conftest
+
+from conftest import _HA_MODULES, _ensure_mock_only
+
+
+def test_mock_only_harness_contract(monkeypatch):
+    """The harness is mock-only: shaped mocks everywhere, fail-fast on real HA."""
+    # Every required homeassistant module in sys.modules is a mock stand-in
+    # with the shaped surface, and parent-package attributes point at them.
+    for name in _HA_MODULES:
+        assert isinstance(sys.modules[name], MagicMock), f"{name} is not a mock"
+
+    config_entries = sys.modules["homeassistant.config_entries"]
+    assert config_entries.ConfigFlow is conftest._ConfigFlowBase
+    assert config_entries.OptionsFlowWithConfigEntry is (
+        conftest._OptionsFlowWithConfigEntryBase
+    )
+    assert sys.modules["homeassistant"].config_entries is config_entries
+    assert sys.modules["homeassistant.core"].callback is conftest._callback_decorator
+    assert sys.modules["homeassistant.data_entry_flow"].RESULT_TYPE_FORM == "form"
+
+    # No homeassistant distribution is installed in this suite's own venv.
+    with pytest.raises(importlib.metadata.PackageNotFoundError):
+        importlib.metadata.version("homeassistant")
+
+    # Fail-fast paths: a findable real HA module, or one preloaded in
+    # sys.modules, both abort collection with a UsageError.
+    real_spec = MagicMock(origin="/fake/site-packages/homeassistant/__init__.py")
+    monkeypatch.setattr(conftest, "_find_spec", lambda name: real_spec)
+    with pytest.raises(pytest.UsageError, match="mock-only"):
+        _ensure_mock_only()
+
+    monkeypatch.setattr(conftest, "_find_spec", lambda name: None)
+    monkeypatch.setitem(sys.modules, "homeassistant", MagicMock(name="real-module"))
+    with pytest.raises(pytest.UsageError, match="mock-only"):
+        _ensure_mock_only()