[api] Fix build error when lambda returns StringRef in homeassistant.event data (#14187)

esphome/components/api/homeassistant_service.h

@@ -36,6 +36,8 @@ template<typename... X> class TemplatableStringValue : public TemplatableValue<s
   static std::string value_to_string(const char *val) { return std::string(val); }  // For lambdas returning .c_str()
   static std::string value_to_string(const std::string &val) { return val; }
   static std::string value_to_string(std::string &&val) { return std::move(val); }
+  static std::string value_to_string(const StringRef &val) { return val.str(); }
+  static std::string value_to_string(StringRef &&val) { return val.str(); }
 
  public:
   TemplatableStringValue() : TemplatableValue<std::string, X...>() {}

tests/components/homeassistant/common.yaml

@@ -90,6 +90,19 @@ text_sensor:
     id: ha_hello_world_text2
     attribute: some_attribute
 
+event:
+  - platform: template
+    name: Test Event
+    id: test_event
+    event_types:
+      - test_event_type
+    on_event:
+      - homeassistant.event:
+          event: esphome.test_event
+          data:
+            event_name: !lambda |-
+              return event_type;
+
 time:
   - platform: homeassistant
     on_time: