feat: Enable setting a default embedding model in the stack (#3803)

# What does this PR do?

Enables automatic embedding model detection for vector stores and by
using a `default_configured` boolean that can be defined in the
`run.yaml`.

<!-- If resolving an issue, uncomment and update the line below -->
<!-- Closes #[issue-number] -->

## Test Plan
- Unit tests
- Integration tests
- Simple example below:

Spin up the stack:
```bash
uv run llama stack build --distro starter --image-type venv --run
```
Then test with OpenAI's client:
```python
from openai import OpenAI
client = OpenAI(base_url="http://localhost:8321/v1/", api_key="none")
vs = client.vector_stores.create()
```
Previously you needed:

```python
vs = client.vector_stores.create(
    extra_body={
        "embedding_model": "sentence-transformers/all-MiniLM-L6-v2",
        "embedding_dimension": 384,
    }
)
```

The `extra_body` is now unnecessary.

---------

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

llama_stack/core/stack.py

@@ -98,6 +98,30 @@ REGISTRY_REFRESH_TASK = None
 TEST_RECORDING_CONTEXT = None
 
 
+async def validate_default_embedding_model(impls: dict[Api, Any]):
+    """Validate that at most one embedding model is marked as default."""
+    if Api.models not in impls:
+        return
+
+    models_impl = impls[Api.models]
+    response = await models_impl.list_models()
+    models_list = response.data if hasattr(response, "data") else response
+
+    default_embedding_models = []
+    for model in models_list:
+        if model.model_type == "embedding" and model.metadata.get("default_configured") is True:
+            default_embedding_models.append(model.identifier)
+
+    if len(default_embedding_models) > 1:
+        raise ValueError(
+            f"Multiple embedding models marked as default_configured=True: {default_embedding_models}. "
+            "Only one embedding model can be marked as default."
+        )
+
+    if default_embedding_models:
+        logger.info(f"Default embedding model configured: {default_embedding_models[0]}")
+
+
 async def register_resources(run_config: StackRunConfig, impls: dict[Api, Any]):
     for rsrc, api, register_method, list_method in RESOURCES:
         objects = getattr(run_config, rsrc)
@@ -128,6 +152,8 @@ async def register_resources(run_config: StackRunConfig, impls: dict[Api, Any]):
                 f"{rsrc.capitalize()}: {obj.identifier} served by {obj.provider_id}",
             )
 
+    await validate_default_embedding_model(impls)
+
 
 class EnvVarError(Exception):
     def __init__(self, var_name: str, path: str = ""):