feat: add custom_metadata to OpenAIModel to unify /v1/models with /v1/openai/v1/models

Goal is to consolidate model listing endpoints. This is step 1: adding custom_metadata
field to OpenAIModel that includes model_type, provider_id, provider_resource_id, and
all model metadata from the native /v1/models response.

Next steps: update stainless client to use /v1/openai/v1/models, migrate tests to read
from custom_metadata, then remove /v1/openai/v1/ prefix entirely.

src/llama_stack/apis/models/models.py

@@ -90,12 +90,14 @@ class OpenAIModel(BaseModel):
     :object: The object type, which will be "model"
     :created: The Unix timestamp in seconds when the model was created
     :owned_by: The owner of the model
+    :custom_metadata: Llama Stack-specific metadata including model_type, provider info, and additional metadata
     """
 
     id: str
     object: Literal["model"] = "model"
     created: int
     owned_by: str
+    custom_metadata: dict[str, Any] | None = None
 
 
 class OpenAIListModelsResponse(BaseModel):

src/llama_stack/core/routing_tables/models.py

@@ -134,6 +134,12 @@ class ModelsRoutingTable(CommonRoutingTableImpl, Models):
                 object="model",
                 created=int(time.time()),
                 owned_by="llama_stack",
+                custom_metadata={
+                    "model_type": model.model_type,
+                    "provider_id": model.provider_id,
+                    "provider_resource_id": model.provider_resource_id,
+                    **model.metadata,
+                },
             )
             for model in all_models
         ]

tests/unit/distribution/routers/test_routing_tables.py

@@ -166,6 +166,14 @@ async def test_models_routing_table(cached_disk_dist_registry):
     assert "test_provider/test-model" in openai_model_ids
     assert "test_provider/test-model-2" in openai_model_ids
 
+    # Verify custom_metadata is populated with Llama Stack-specific data
+    for openai_model in openai_models.data:
+        assert openai_model.custom_metadata is not None
+        assert "model_type" in openai_model.custom_metadata
+        assert "provider_id" in openai_model.custom_metadata
+        assert "provider_resource_id" in openai_model.custom_metadata
+        assert openai_model.custom_metadata["provider_id"] == "test_provider"
+
     # Test get_object_by_identifier
     model = await table.get_object_by_identifier("model", "test_provider/test-model")
     assert model is not None