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/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
         ]