feat: Add configurable thinking indicator text

- Add thinkingIndicatorText field to interface configuration schema
- Update ContentParts component to use configurable text instead of hardcoded translation
- Update example config with new option
- Maintain backward compatibility by falling back to default translation

packages/data-provider/src/config.ts

@@ -550,6 +550,7 @@ export const interfaceSchema = z
       .optional(),
     fileSearch: z.boolean().optional(),
     fileCitations: z.boolean().optional(),
+    thinkingIndicatorText: z.string().optional(),
   })
   .default({
     endpointsMenu: true,