[core] Use StringRef for get_comment and get_compilation_time to avoid allocations

esphome/core/helpers.h

@@ -15,6 +15,7 @@
 #include <concepts>
 
 #include "esphome/core/optional.h"
+#include "esphome/core/string_ref.h"
 
 #ifdef USE_ESP8266
 #include <Esp.h>
@@ -47,9 +48,6 @@
 
 namespace esphome {
 
-// Forward declaration to avoid circular dependency with string_ref.h
-class StringRef;
-
 /// @name STL backports
 ///@{