feat: add custom_metadata to OpenAIModel to unify /v1/models with /v1/openai/v1/models (#4051)

We need to remove `/v1/openai/v1` paths shortly. There is one trouble --
our current `/v1/openai/v1/models` endpoint provides different data than
`/v1/models`. Unfortunately our tests target the latter (llama-stack
customized) behavior. We need to get to true OpenAI compatibility.

This is step 1: adding `custom_metadata` field to `OpenAIModel` that
includes all the extra stuff we add in the native `/v1/models` response.
This can be extracted on the consumer end by look at
`__pydantic_extra__` or other similar fields.

This PR:
- Adds `custom_metadata` field to `OpenAIModel` class in
`src/llama_stack/apis/models/models.py`
- Modified `openai_list_models()` in
`src/llama_stack/core/routing_tables/models.py` to populate
custom_metadata

Next Steps
1. Update stainless client to use `/v1/openai/v1/models` instead of
`/v1/models`
2. Migrate tests to read from `custom_metadata`
3. Remove `/v1/openai/v1/` prefix entirely and consolidate to single
`/v1/models` endpoint

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):
@@ -113,7 +115,7 @@ class Models(Protocol):
         """
         ...
 
-    @webmethod(route="/openai/v1/models", method="GET", level=LLAMA_STACK_API_V1, deprecated=True)
+    @webmethod(route="/openai/v1/models", method="GET", level=LLAMA_STACK_API_V1)
     async def openai_list_models(self) -> OpenAIListModelsResponse:
         """List models using the OpenAI API.
 

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
         ]