[core] Inline HighFrequencyLoopRequester::is_high_frequency()

Move is_high_frequency() from out-of-line definition in helpers.cpp
to inline in the header. This allows the compiler to inline the
trivial check (num_requests > 0) at the call site in
Application::loop(), avoiding a function call every loop iteration.

esphome/core/helpers.cpp

@@ -794,7 +794,6 @@ void HighFrequencyLoopRequester::stop() {
   num_requests--;
   this->started_ = false;
 }
-bool HighFrequencyLoopRequester::is_high_frequency() { return num_requests > 0; }
 
 std::string get_mac_address() {
   uint8_t mac[6];

esphome/core/helpers.h

@@ -1732,7 +1732,7 @@ class HighFrequencyLoopRequester {
   void stop();
 
   /// Check whether the loop is running continuously.
-  static bool is_high_frequency();
+  static bool is_high_frequency() { return num_requests > 0; }
 
  protected:
   bool started_{false};