[safe_mode] Use StaticCallbackManager for on_safe_mode

safe_mode is a single-instance component, so a compile-time-sized
StaticVector backing for the callback list is a clean fit. Drop the
std::vector reallocation machinery (_M_realloc_insert) and size the
storage to the exact number of on_safe_mode automations via
ESPHOME_SAFE_MODE_CALLBACK_COUNT.

esphome/components/safe_mode/__init__.py

@@ -76,8 +76,9 @@ async def to_code(config):
         var = cg.new_Pvariable(config[CONF_ID])
         await cg.register_component(var, config)
 
-        if config.get(CONF_ON_SAFE_MODE):
+        if on_safe_mode := config.get(CONF_ON_SAFE_MODE):
             cg.add_define("USE_SAFE_MODE_CALLBACK")
+            cg.add_define("ESPHOME_SAFE_MODE_CALLBACK_COUNT", len(on_safe_mode))
             await automation.build_callback_automations(
                 var, config, _CALLBACK_AUTOMATIONS
             )

esphome/components/safe_mode/safe_mode.h

@@ -57,7 +57,7 @@ class SafeModeComponent final : public Component {
   // Larger objects at the end
   ESPPreferenceObject rtc_;
 #ifdef USE_SAFE_MODE_CALLBACK
-  CallbackManager<void()> safe_mode_callback_{};
+  StaticCallbackManager<ESPHOME_SAFE_MODE_CALLBACK_COUNT, void()> safe_mode_callback_{};
 #endif
 
   static const uint32_t ENTER_SAFE_MODE_MAGIC =

esphome/core/defines.h

@@ -136,6 +136,7 @@
 #define USE_PREFERENCES_SYNC_EVERY_LOOP
 #define USE_QR_CODE
 #define USE_SAFE_MODE_CALLBACK
+#define ESPHOME_SAFE_MODE_CALLBACK_COUNT 1
 #define USE_SELECT
 #define USE_SENSOR
 #define USE_SENSOR_FILTER