[web_server_idf] Use make_unique_for_overwrite for recv buffer

The multipart receive buffer is immediately written by httpd_req_recv()
before any read occurs, and only recv_len bytes (the amount actually
received) are subsequently passed to the parser. Zero-initialization
via make_unique is therefore unnecessary overhead.

Switch to C++20 make_unique_for_overwrite which skips value-initialization
for POD types, avoiding a redundant memset of the 1460-byte buffer on
every multipart upload request.

esphome/components/web_server_idf/web_server_idf.cpp

@@ -921,7 +921,7 @@ esp_err_t AsyncWebServer::handle_multipart_upload_(httpd_req_t *r, const char *c
   });
 
   // Use heap buffer - 1460 bytes is too large for the httpd task stack
-  auto buffer = std::make_unique<char[]>(MULTIPART_CHUNK_SIZE);
+  auto buffer = std::make_unique_for_overwrite<char[]>(MULTIPART_CHUNK_SIZE);
   size_t bytes_since_yield = 0;
 
   for (size_t remaining = r->content_len; remaining > 0;) {