[core] cpp tests: Allow customizing code generation during tests (#14681)

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

tests/testing_helpers.py

@@ -0,0 +1,63 @@
+from typing import Any
+
+from esphome.loader import ComponentManifest, _replace_component_manifest
+
+
+class ComponentManifestOverride:
+    """Mutable wrapper around ComponentManifest for test-specific attribute overrides.
+
+    When ``tests/components/<name>/__init__.py`` defines::
+
+        def override_manifest(manifest: ComponentManifestOverride) -> None:
+            ...
+
+    the function receives an instance of this class wrapping the real component
+    manifest.  Any attribute assignment stores an override; reads fall back to
+    the underlying ``ComponentManifest`` when no override has been set.
+
+    Example::
+
+        def override_manifest(manifest: ComponentManifestOverride) -> None:
+            async def to_code_testing(config):
+                pass  # lightweight no-op stub for C++ unit tests
+
+            manifest.to_code = to_code_testing
+            manifest.dependencies = manifest.dependencies + ["extra_dep_for_tests"]
+    """
+
+    def __init__(self, wrapped: "ComponentManifest") -> None:
+        object.__setattr__(self, "_wrapped", wrapped)
+        object.__setattr__(self, "_overrides", {})
+
+    def __getattr__(self, name: str) -> Any:
+        overrides: dict[str, Any] = object.__getattribute__(self, "_overrides")
+        if name in overrides:
+            return overrides[name]
+        wrapped: ComponentManifest = object.__getattribute__(self, "_wrapped")
+        return getattr(wrapped, name)
+
+    def __setattr__(self, name: str, value: Any) -> None:
+        overrides: dict[str, Any] = object.__getattribute__(self, "_overrides")
+        overrides[name] = value
+
+    def enable_codegen(self) -> None:
+        """Remove the to_code suppression, re-enabling code generation for this component.
+
+        Call this from ``override_manifest`` when the component needs its real (or a
+        custom stub) ``to_code`` to run during C++ unit test builds.
+        """
+        overrides: dict[str, Any] = object.__getattribute__(self, "_overrides")
+        overrides.pop("to_code", None)
+
+    def restore(self) -> None:
+        """Clear all overrides, reverting to the wrapped manifest's values."""
+        object.__getattribute__(self, "_overrides").clear()
+
+
+def set_testing_manifest(domain: str, manifest: ComponentManifestOverride) -> None:
+    """Install a testing manifest override into the component cache.
+
+    Called from the C++ unit test infrastructure when a component's test
+    directory provides an ``override_manifest`` function.
+    """
+    _replace_component_manifest(domain, manifest)