Handle Oauth2 ImplementationUnavailableError in gentex_homelink (#166646)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/components/gentex_homelink/test_init.py

@@ -8,6 +8,9 @@ from syrupy.assertion import SnapshotAssertion
 from homeassistant.components.gentex_homelink.const import DOMAIN
 from homeassistant.config_entries import ConfigEntryState
 from homeassistant.core import HomeAssistant
+from homeassistant.helpers.config_entry_oauth2_flow import (
+    ImplementationUnavailableError,
+)
 import homeassistant.helpers.device_registry as dr
 
 from . import setup_integration, update_callback
@@ -72,3 +75,21 @@ async def test_load_unload_entry(
     await hass.async_block_till_done()
 
     assert mock_config_entry.state is ConfigEntryState.NOT_LOADED
+
+
+@pytest.mark.usefixtures("aioclient_mock_fixture")
+async def test_oauth_implementation_not_available(
+    hass: HomeAssistant,
+    mock_config_entry: MockConfigEntry,
+) -> None:
+    """Test that unavailable OAuth implementation raises ConfigEntryNotReady."""
+    mock_config_entry.add_to_hass(hass)
+
+    with patch(
+        "homeassistant.helpers.config_entry_oauth2_flow.async_get_config_entry_implementation",
+        side_effect=ImplementationUnavailableError,
+    ):
+        await hass.config_entries.async_setup(mock_config_entry.entry_id)
+        await hass.async_block_till_done()
+
+    assert mock_config_entry.state is ConfigEntryState.SETUP_RETRY