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

.github/workflows/integration-vector-io-tests.yml

@@ -114,7 +114,7 @@ jobs:
         run: |
           uv run pytest -sv --stack-config="inference=inline::sentence-transformers,vector_io=${{ matrix.vector-io-provider }}" \
             tests/integration/vector_io \
-            --embedding-model all-MiniLM-L6-v2
+            --embedding-model sentence-transformers/all-MiniLM-L6-v2
 
       - name: Check Storage and Memory Available After Tests
         if: ${{ always() }}