Expose "Allow fallback providers" for OpenRouter

src/endpoints/backends/text-completions.js

@@ -309,7 +309,7 @@ router.post('/generate', jsonParser, async function (request, response) {
         if (request.body.api_type === TEXTGEN_TYPES.OPENROUTER) {
             if (Array.isArray(request.body.provider) && request.body.provider.length > 0) {
                 request.body.provider = {
-                    allow_fallbacks: true,
+                    allow_fallbacks: request.body.allow_fallbacks ?? true,
                     order: request.body.provider,
                 };
             } else {