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

scripts/integration-tests.sh

@@ -290,7 +290,7 @@ pytest -s -v $PYTEST_TARGET \
     -k "$PYTEST_PATTERN" \
     $EXTRA_PARAMS \
     --color=yes \
-    --embedding-model=nomic-ai/nomic-embed-text-v1.5 \
+    --embedding-model=sentence-transformers/nomic-ai/nomic-embed-text-v1.5 \
     --color=yes $EXTRA_PARAMS \
     --capture=tee-sys
 exit_code=$?