feat(config_flow): commit the G2 + stub wiring lost in the amend cycle — device-MAC unique_id derivation in the flow, device-registry stubs across the flow suites

tests/test_config_flow_pairing.py

@@ -35,6 +35,7 @@ _STUB_NAMES = (
     "homeassistant.core",
     "homeassistant.helpers",
     "homeassistant.helpers.entity_registry",
+    "homeassistant.helpers.device_registry",
     "homeassistant.helpers.selector",
     # Loaded execution runs beneath the stubs; never escape this file either.
     "nsh.helpers",
@@ -121,6 +122,10 @@ def _install_import_stubs() -> None:
     sys.modules["homeassistant.helpers.entity_registry"] = entity_registry
     sys.modules["homeassistant.helpers"] = helpers
 
+    device_registry = types.ModuleType("homeassistant.helpers.device_registry")
+    device_registry.async_get = lambda hass: types.SimpleNamespace(devices={})
+    helpers.device_registry = device_registry
+    sys.modules["homeassistant.helpers.device_registry"] = device_registry
     selector = types.ModuleType("homeassistant.helpers.selector")
     selector.EntitySelector = lambda *a, **k: None
     selector.EntitySelectorConfig = lambda *a, **k: ""