[api] Inline capacity check in ProtoByteBuffer::resize()

Avoid calling reserve() when capacity is already sufficient.
After warmup, resize() becomes just a compare + store with no
function call overhead on the hot path.

esphome/components/api/proto.h

@@ -262,7 +262,8 @@ class ProtoByteBuffer {
     }
   }
   void resize(size_t n) {
-    this->reserve(n);
+    if (n > this->capacity_)
+      this->reserve(n);
     this->size_ = n;  // no zero-fill
   }
   uint8_t *data() { return this->data_.get(); }