[api] Outline ProtoByteBuffer reallocation into grow_()

The capacity check in reserve()/resize() is the hot path and stays
inline. The actual reallocation (make_buffer + memcpy) is a cold path
that should be outlined to avoid bloating every call site.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

esphome/components/api/proto.cpp

@@ -8,6 +8,14 @@ namespace esphome::api {
 
 static const char *const TAG = "api.proto";
 
+void ProtoByteBuffer::grow_(size_t n) {
+  auto new_data = make_buffer(n);
+  if (this->size_)
+    std::memcpy(new_data.get(), this->data_.get(), this->size_);
+  this->data_ = std::move(new_data);
+  this->capacity_ = n;
+}
+
 #ifdef USE_API_VARINT64
 optional<ProtoVarInt> ProtoVarInt::parse_wide(const uint8_t *buffer, uint32_t len, uint32_t *consumed,
                                               uint32_t result32) {

esphome/components/api/proto.h

@@ -260,17 +260,12 @@ class ProtoByteBuffer {
  public:
   void clear() { this->size_ = 0; }
   void reserve(size_t n) {
-    if (n > this->capacity_) {
-      auto new_data = make_buffer(n);
-      if (this->size_)
-        std::memcpy(new_data.get(), this->data_.get(), this->size_);
-      this->data_ = std::move(new_data);
-      this->capacity_ = n;
-    }
+    if (n > this->capacity_)
+      this->grow_(n);
   }
   void resize(size_t n) {
     if (n > this->capacity_)
-      this->reserve(n);
+      this->grow_(n);
     this->size_ = n;  // no zero-fill
   }
   uint8_t *data() { return this->data_.get(); }
@@ -278,6 +273,7 @@ class ProtoByteBuffer {
   size_t size() const { return this->size_; }
 
  protected:
+  void grow_(size_t n);
   std::unique_ptr<uint8_t[]> data_;
   size_t size_{0};
   size_t capacity_{0};