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>

tests/unit/core/test_stack_validation.py

@@ -0,0 +1,93 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+"""
+Unit tests for Stack validation functions.
+"""
+
+from unittest.mock import AsyncMock
+
+import pytest
+
+from llama_stack.apis.models import Model, ModelType
+from llama_stack.core.stack import validate_default_embedding_model
+from llama_stack.providers.datatypes import Api
+
+
+class TestStackValidation:
+    """Test Stack validation functions."""
+
+    @pytest.mark.parametrize(
+        "models,should_raise",
+        [
+            ([], False),  # No models
+            (
+                [
+                    Model(
+                        identifier="emb1",
+                        model_type=ModelType.embedding,
+                        metadata={"default_configured": True},
+                        provider_id="p",
+                        provider_resource_id="emb1",
+                    )
+                ],
+                False,
+            ),  # Single default
+            (
+                [
+                    Model(
+                        identifier="emb1",
+                        model_type=ModelType.embedding,
+                        metadata={"default_configured": True},
+                        provider_id="p",
+                        provider_resource_id="emb1",
+                    ),
+                    Model(
+                        identifier="emb2",
+                        model_type=ModelType.embedding,
+                        metadata={"default_configured": True},
+                        provider_id="p",
+                        provider_resource_id="emb2",
+                    ),
+                ],
+                True,
+            ),  # Multiple defaults
+            (
+                [
+                    Model(
+                        identifier="emb1",
+                        model_type=ModelType.embedding,
+                        metadata={"default_configured": True},
+                        provider_id="p",
+                        provider_resource_id="emb1",
+                    ),
+                    Model(
+                        identifier="llm1",
+                        model_type=ModelType.llm,
+                        metadata={"default_configured": True},
+                        provider_id="p",
+                        provider_resource_id="llm1",
+                    ),
+                ],
+                False,
+            ),  # Ignores non-embedding
+        ],
+    )
+    async def test_validate_default_embedding_model(self, models, should_raise):
+        """Test validation with various model configurations."""
+        mock_models_impl = AsyncMock()
+        mock_models_impl.list_models.return_value = models
+        impls = {Api.models: mock_models_impl}
+
+        if should_raise:
+            with pytest.raises(ValueError, match="Multiple embedding models marked as default_configured=True"):
+                await validate_default_embedding_model(impls)
+        else:
+            await validate_default_embedding_model(impls)
+
+    async def test_validate_default_embedding_model_no_models_api(self):
+        """Test validation when models API is not available."""
+        await validate_default_embedding_model({})