server : do not parse when flushing http headers (#24281)

tools/server/server-task.cpp

@@ -1393,6 +1393,9 @@ json server_task_result_cmpl_final::to_json_anthropic_stream() {
 //
 void server_task_result_cmpl_partial::update(task_result_state & state) {
     is_updated = true;
+    if (is_begin) {
+        return; // begin marker only flushes headers, skip parsing
+    }
     state.update_chat_msg(content, true, oaicompat_msg_diffs);
 
     // Copy current state for use in to_json_*() (reflects state BEFORE this chunk)