feat(registry): more flexible model lookup (#2859)

This PR updates model registration and lookup behavior to be slightly
more general / flexible. See
https://github.com/meta-llama/llama-stack/issues/2843 for more details.

Note that this change is backwards compatible given the design of the
`lookup_model()` method.

## Test Plan

Added unit tests

llama_stack/providers/utils/inference/embedding_mixin.py

@@ -88,7 +88,7 @@ class SentenceTransformerEmbeddingMixin:
         usage = OpenAIEmbeddingUsage(prompt_tokens=-1, total_tokens=-1)
         return OpenAIEmbeddingsResponse(
             data=data,
-            model=model_obj.provider_resource_id,
+            model=model,
             usage=usage,
         )