fix(ai): return CURL_WRITEFUNC_ERROR on oversized response

Return CURL_WRITEFUNC_ERROR instead of realsize when the response
exceeds 10MB. This causes curl to abort the transfer immediately
instead of continuing to stream data.

src/ai/ai_client.c

@@ -50,8 +50,8 @@ _write_callback(void* ptr, size_t size, size_t nmemb, void* userdata)
 
     /* Limit response size to 10MB to prevent OOM */
     if (res->size + realsize > 10 * 1024 * 1024) {
-        log_error("[AI-THREAD] Response too large, truncating");
-        return realsize;
+        log_error("[AI-THREAD] Response too large, aborting transfer");
+        return CURL_WRITEFUNC_ERROR;
     }
 
     gchar* new_data = g_realloc(res->data, res->size + realsize + 1);