core/cli: soft-fail GET /models cancel/timeout on provider switch

Map CancelledError and TimeoutError from models() to RuntimeError so
merged_model_choices falls back to config/cache. Stub remote fetch in
/provider unit tests so they never hit the network.

tests/test_cli/test_models_source.py

@@ -67,7 +67,7 @@ async def test_fetch_remote_model_ids_timeout() -> None:
             await asyncio.sleep(10)
             return ["late"]
 
-    with pytest.raises(TimeoutError):
+    with pytest.raises(RuntimeError, match="timed out"):
         _ = await fetch_remote_model_ids(Slow(), timeout_seconds=0.05)