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-tests.yml

@@ -99,7 +99,7 @@ jobs:
           uv run pytest -s -v tests/integration/${{ matrix.test-type }} --stack-config=${stack_config} \
             -k "not(builtin_tool or safety_with_image or code_interpreter or test_rag)" \
             --text-model="ollama/llama3.2:3b-instruct-fp16" \
-            --embedding-model=all-MiniLM-L6-v2 \
+            --embedding-model=sentence-transformers/all-MiniLM-L6-v2 \
             --safety-shield=$SAFETY_MODEL \
             --color=yes \
             --capture=tee-sys | tee pytest-${{ matrix.test-type }}.log