[binary_sensor] Move set_trigger_on_initial_state off StatefulEntityBase

set_trigger_on_initial_state is only called from codegen on concrete
BinarySensor instances. It's an implementation detail of BinarySensor,
not part of the StatefulEntityBase contract. Only get_trigger_on_initial_state
remains as a pure virtual — subclasses decide how to control it.

esphome/components/binary_sensor/binary_sensor.h

@@ -35,7 +35,7 @@ class BinarySensor : public StatefulEntityBase<bool> {
   explicit BinarySensor() = default;
 
   const bool &get_state() const override { return this->state; }
-  void set_trigger_on_initial_state(bool value) override { this->trigger_on_initial_state_ = value; }
+  void set_trigger_on_initial_state(bool value) { this->trigger_on_initial_state_ = value; }
 
   /** Publish a new state to the front-end.
    *

esphome/core/entity_base.h

@@ -305,7 +305,6 @@ void log_entity_unit_of_measurement(const char *tag, const char *prefix, const E
  *   - get_state(): return a const reference to the current value
  *   - set_state_value(): store a new value (called only when the state actually changes)
  *   - get_trigger_on_initial_state(): return whether callbacks should fire on the first state
- *   - set_trigger_on_initial_state(): store the value (subclass decides how)
  *
  * Subclasses may override set_new_state() to add behavior (logging, notifications) after calling
  * the base implementation. Since set_new_state() is virtual, callers like invalidate_state() and
@@ -335,10 +334,6 @@ template<typename T> class StatefulEntityBase : public EntityBase {
     this->state_callbacks_.add(std::forward<F>(callback));
   }
 
-  /// Control whether state_callbacks_ fire on the very first state (before any previous state exists).
-  /// Subclasses decide how to store the value.
-  virtual void set_trigger_on_initial_state(bool value) = 0;
-
  protected:
   /// Subclasses return whether callbacks should fire on the very first state.
   virtual bool get_trigger_on_initial_state() const = 0;