fix(inference): enable routing of models with provider_data alone

Assume a remote inference provider which works only when users provide
their own API keys via provider_data. By definition, we cannot list
models and hence update our routing registry. But because we _require_ a
provider ID in the models now, we can identify which provider to route
to and let that provider decide.

Note that we still try to look up our registry since it may have a
pre-registered alias. Just that we don't outright fail when we are not
able to look it up.

Also, updated inference router so that the responses have the _exact_
model that the request had.

Added an integration test

tests/integration/telemetry/test_completions.py

@@ -64,7 +64,7 @@ def test_telemetry_format_completeness(mock_otlp_collector, llama_stack_client,
 
     # Verify spans
     spans = mock_otlp_collector.get_spans()
-    assert len(spans) == 5
+    assert len(spans) == 5, f"Expected 5 spans, got {len(spans)}"
 
     # we only need this captured one time
     logged_model_id = None