[api] Implement zero-copy API for bluetooth_proxy writes (#10840)

esphome/components/api/proto.h

@@ -182,6 +182,10 @@ class ProtoLengthDelimited {
   explicit ProtoLengthDelimited(const uint8_t *value, size_t length) : value_(value), length_(length) {}
   std::string as_string() const { return std::string(reinterpret_cast<const char *>(this->value_), this->length_); }
 
+  // Direct access to raw data without string allocation
+  const uint8_t *data() const { return this->value_; }
+  size_t size() const { return this->length_; }
+
   /**
    * Decode the length-delimited data into an existing ProtoDecodableMessage instance.
    *