[cpptests] support testing platform components (#13075)

Co-authored-by: J. Nick Koston <nick@home-assistant.io>
Co-authored-by: J. Nick Koston <nick@koston.org>

tests/script/test_helpers.py

@@ -1027,6 +1027,73 @@ def test_get_all_dependencies_empty_set() -> None:
     assert result == set()
 
 
+def test_get_all_dependencies_platform_component() -> None:
+    """Platform components (domain.component) are looked up via get_platform,
+    not get_component."""
+    platform_comp = Mock()
+    platform_comp.dependencies = []
+    platform_comp.auto_load = []
+
+    with (
+        patch("esphome.loader.get_component") as mock_get_component,
+        patch("helpers.get_platform") as mock_get_platform,
+    ):
+        mock_get_platform.return_value = platform_comp
+        mock_get_component.return_value = None
+
+        result = helpers.get_all_dependencies({"sensor.bthome"})
+
+        mock_get_platform.assert_called_once_with("sensor", "bthome")
+        mock_get_component.assert_not_called()
+        assert result == {"sensor.bthome"}
+
+
+def test_get_all_dependencies_platform_component_with_dependencies() -> None:
+    """Dependencies of a platform component are resolved transitively."""
+    platform_comp = Mock()
+    platform_comp.dependencies = ["sensor"]
+    platform_comp.auto_load = []
+
+    sensor_comp = Mock()
+    sensor_comp.dependencies = []
+    sensor_comp.auto_load = []
+
+    with (
+        patch("esphome.loader.get_component") as mock_get_component,
+        patch("helpers.get_platform") as mock_get_platform,
+    ):
+        mock_get_platform.return_value = platform_comp
+        mock_get_component.side_effect = lambda name: (
+            sensor_comp if name == "sensor" else None
+        )
+
+        result = helpers.get_all_dependencies({"sensor.bthome"})
+
+        assert result == {"sensor.bthome", "sensor"}
+
+
+def test_get_all_dependencies_cpp_testing_flag() -> None:
+    """cpp_testing=True propagates to CORE.cpp_testing during resolution."""
+    from esphome.core import CORE
+
+    with (
+        patch("esphome.loader.get_component") as mock_get_component,
+        patch("esphome.loader.get_platform"),
+    ):
+        observed: list[bool] = []
+
+        def capturing_get_component(name: str):
+            observed.append(CORE.cpp_testing)
+
+        mock_get_component.side_effect = capturing_get_component
+
+        helpers.get_all_dependencies({"some_comp"}, cpp_testing=True)
+
+    assert observed and all(observed), (
+        "CORE.cpp_testing should be True during resolution"
+    )
+
+
 def test_get_components_from_integration_fixtures() -> None:
     """Test extraction of components from fixture YAML files."""
     yaml_content = {