[api] Inline DeferredBatch::push_item to avoid call overhead

push_item was defined out-of-line in the .cpp to avoid duplicate
_M_realloc_insert instantiation, but the function call overhead
on every add_item was visible in benchmarks. Move it inline since
it's a one-liner wrapper around push_back.

esphome/components/api/api_connection.cpp

@@ -2072,8 +2072,6 @@ void APIConnection::on_fatal_error() {
   this->flags_.remove = true;
 }
 
-void __attribute__((flatten)) APIConnection::DeferredBatch::push_item(const BatchItem &item) { items.push_back(item); }
-
 void APIConnection::DeferredBatch::add_item(EntityBase *entity, uint8_t message_type, uint8_t estimated_size,
                                             uint8_t aux_data_index) {
   // Check if we already have a message of this type for this entity

esphome/components/api/api_connection.h

@@ -647,8 +647,7 @@ class APIConnection final : public APIServerConnectionBase {
                   uint8_t aux_data_index = AUX_DATA_UNUSED);
     // Add item to the front of the batch (for high priority messages like ping)
     void add_item_front(EntityBase *entity, uint8_t message_type, uint8_t estimated_size);
-    // Single push_back site to avoid duplicate _M_realloc_insert instantiation
-    void push_item(const BatchItem &item);
+    void push_item(const BatchItem &item) { items.push_back(item); }
 
     // Clear all items
     void clear() {