fix(models)!: always prefix models with provider_id when registering (#3822)

**!!BREAKING CHANGE!!**

The lookup is also straightforward -- we always look for this identifier
and don't try to find a match for something without the provider_id
prefix.

Note that, this ideally means we need to update the `register_model()`
API also (we should kill "identifier" from there) but I am not doing
that as part of this PR.

## Test Plan

Existing unit tests

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

@@ -170,7 +170,7 @@ jobs:
           uv run --no-sync \
             pytest -sv --stack-config="files=inline::localfs,inference=inline::sentence-transformers,vector_io=${{ matrix.vector-io-provider }}" \
             tests/integration/vector_io \
-            --embedding-model nomic-ai/nomic-embed-text-v1.5 \
+            --embedding-model inline::sentence-transformers/nomic-ai/nomic-embed-text-v1.5 \
             --embedding-dimension 768
 
       - name: Check Storage and Memory Available After Tests