feat(telegram): paginated model picker with Next/Prev navigation

- Raise max_models from 8 to 50 so all curated models come through
- Add _build_model_keyboard() helper with 8-per-page pagination
- Next ▶ / ◀ Prev buttons with page counter (e.g. 2/4)
- mg:<page> callback data for page navigation
- Catch-all query.answer() for noop buttons

gateway/run.py

@@ -3530,7 +3530,7 @@ class GatewayRunner:
                     providers = list_authenticated_providers(
                         current_provider=current_provider,
                         user_providers=user_provs,
-                        max_models=8,
+                        max_models=50,
                     )
                 except Exception:
                     providers = []