Add static_asserts for forwarder size, widen type hint

- static_assert that forwarders are pointer-sized and trivially
  copyable so future field additions can't silently cause heap
  allocation in Callback::create().
- Widen forwarder parameter type hint to MockObj | MockObjClass.

esphome/automation.py

@@ -671,7 +671,7 @@ async def build_callback_automation(
     callback_method: str,
     args: TemplateArgsType,
     config: ConfigType,
-    forwarder: MockObjClass | None = None,
+    forwarder: MockObj | MockObjClass | None = None,
 ) -> None:
     """Build an Automation and register it as a callback on the parent.
 

esphome/core/automation.h

@@ -494,12 +494,14 @@ template<typename... Ts> class Automation {
 
 /// Callback forwarder that triggers an Automation directly.
 /// One operator() instantiation per Automation<Ts...> signature, shared across all call sites.
+/// Must stay pointer-sized to fit inline in Callback::ctx_ without heap allocation.
 template<typename... Ts> struct TriggerForwarder {
   Automation<Ts...> *automation;
   void operator()(const Ts &...args) const { this->automation->trigger(args...); }
 };
 
 /// Callback forwarder that triggers an Automation<> only when the bool arg is true.
+/// Must stay pointer-sized to fit inline in Callback::ctx_ without heap allocation.
 struct TriggerOnTrueForwarder {
   Automation<> *automation;
   void operator()(bool state) const {
@@ -509,6 +511,7 @@ struct TriggerOnTrueForwarder {
 };
 
 /// Callback forwarder that triggers an Automation<> only when the bool arg is false.
+/// Must stay pointer-sized to fit inline in Callback::ctx_ without heap allocation.
 struct TriggerOnFalseForwarder {
   Automation<> *automation;
   void operator()(bool state) const {
@@ -517,4 +520,13 @@ struct TriggerOnFalseForwarder {
   }
 };
 
+// Ensure forwarders fit in Callback::ctx_ (pointer-sized inline storage).
+// If these fail, the forwarder would heap-allocate in Callback::create().
+static_assert(sizeof(TriggerForwarder<>) <= sizeof(void *));
+static_assert(sizeof(TriggerOnTrueForwarder) <= sizeof(void *));
+static_assert(sizeof(TriggerOnFalseForwarder) <= sizeof(void *));
+static_assert(std::is_trivially_copyable_v<TriggerForwarder<>>);
+static_assert(std::is_trivially_copyable_v<TriggerOnTrueForwarder>);
+static_assert(std::is_trivially_copyable_v<TriggerOnFalseForwarder>);
+
 }  // namespace esphome