chore: Auto-detect Provider ID when only 1 Vector Store Provider avai… (#3802)

# What does this PR do?
2 main changes:

1. Remove `provider_id` requirement in call to vector stores and
2. Removes "register first embedding model" logic 
   - Now forces embedding model id as required on Vector Store creation

Simplifies the UX for OpenAI to:

```python
vs = client.vector_stores.create(
    name="my_citations_db",
    extra_body={
        "embedding_model": "ollama/nomic-embed-text:latest",
    }
)
```


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## Test Plan
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---------

Signed-off-by: Francisco Javier Arceo <farceo@redhat.com>

llama_stack/providers/utils/memory/openai_vector_store_mixin.py

@@ -353,14 +353,12 @@ class OpenAIVectorStoreMixin(ABC):
         provider_vector_db_id = extra.get("provider_vector_db_id")
         embedding_model = extra.get("embedding_model")
         embedding_dimension = extra.get("embedding_dimension", 384)
-        provider_id = extra.get("provider_id")
+        # use provider_id set by router; fallback to provider's own ID when used directly via --stack-config
+        provider_id = extra.get("provider_id") or getattr(self, "__provider_id__", None)
 
         # Derive the canonical vector_db_id (allow override, else generate)
         vector_db_id = provider_vector_db_id or generate_object_id("vector_store", lambda: f"vs_{uuid.uuid4()}")
 
-        if provider_id is None:
-            raise ValueError("Provider ID is required")
-
         if embedding_model is None:
             raise ValueError("Embedding model is required")
 
@@ -369,6 +367,9 @@ class OpenAIVectorStoreMixin(ABC):
             raise ValueError("Embedding dimension is required")
 
         # Register the VectorDB backing this vector store
+        if provider_id is None:
+            raise ValueError("Provider ID is required but was not provided")
+
         vector_db = VectorDB(
             identifier=vector_db_id,
             embedding_dimension=embedding_dimension,