fix: enforce allowed_models during inference requests

The `allowed_models` configuration was only filtering the model list endpoint but not enforcing restrictions during actual inference requests. This allowed users to bypass the restriction by directly requesting models not in the allowed list, potentially accessing expensive models when only cheaper ones were intended.

This change adds validation to all inference methods (`openai_chat_completion`, `openai_completion`, `openai_embeddings`) to reject requests for disallowed models with a clear error message.

**Implementation:**
- Added `_validate_model_allowed()` helper method that checks if a model is in the `allowed_models` list
- Called validation in all three inference methods before making API requests
- Validation occurs after resolving the provider model ID to ensure consistency

**Test Plan:**
- Added unit tests verifying all inference methods respect `allowed_models`
- Tests cover allowed models (success), disallowed models (rejection), and no restrictions (None allows all, empty list blocks all)
- All existing tests continue to pass

Fixes GHSA-5rjj-4jp6-fw39

src/llama_stack/providers/utils/inference/openai_mixin.py

@@ -213,6 +213,19 @@ class OpenAIMixin(NeedsRequestProviderData, ABC, BaseModel):
 
         return api_key
 
+    def _validate_model_allowed(self, provider_model_id: str) -> None:
+        """
+        Validate that the model is in the allowed_models list if configured.
+
+        :param provider_model_id: The provider-specific model ID to validate
+        :raises ValueError: If the model is not in the allowed_models list
+        """
+        if self.config.allowed_models is not None and provider_model_id not in self.config.allowed_models:
+            raise ValueError(
+                f"Model '{provider_model_id}' is not in the allowed models list. "
+                f"Allowed models: {self.config.allowed_models}"
+            )
+
     async def _get_provider_model_id(self, model: str) -> str:
         """
         Get the provider-specific model ID from the model store.
@@ -259,8 +272,11 @@ class OpenAIMixin(NeedsRequestProviderData, ABC, BaseModel):
         Direct OpenAI completion API call.
         """
         # TODO: fix openai_completion to return type compatible with OpenAI's API response
+        provider_model_id = await self._get_provider_model_id(params.model)
+        self._validate_model_allowed(provider_model_id)
+
         completion_kwargs = await prepare_openai_completion_params(
-            model=await self._get_provider_model_id(params.model),
+            model=provider_model_id,
             prompt=params.prompt,
             best_of=params.best_of,
             echo=params.echo,
@@ -292,6 +308,9 @@ class OpenAIMixin(NeedsRequestProviderData, ABC, BaseModel):
         """
         Direct OpenAI chat completion API call.
         """
+        provider_model_id = await self._get_provider_model_id(params.model)
+        self._validate_model_allowed(provider_model_id)
+
         messages = params.messages
 
         if self.download_images:
@@ -313,7 +332,7 @@ class OpenAIMixin(NeedsRequestProviderData, ABC, BaseModel):
             messages = [await _localize_image_url(m) for m in messages]
 
         request_params = await prepare_openai_completion_params(
-            model=await self._get_provider_model_id(params.model),
+            model=provider_model_id,
             messages=messages,
             frequency_penalty=params.frequency_penalty,
             function_call=params.function_call,
@@ -351,10 +370,13 @@ class OpenAIMixin(NeedsRequestProviderData, ABC, BaseModel):
         """
         Direct OpenAI embeddings API call.
         """
+        provider_model_id = await self._get_provider_model_id(params.model)
+        self._validate_model_allowed(provider_model_id)
+
         # Build request params conditionally to avoid NotGiven/Omit type mismatch
         # The OpenAI SDK uses Omit in signatures but NOT_GIVEN has type NotGiven
         request_params: dict[str, Any] = {
-            "model": await self._get_provider_model_id(params.model),
+            "model": provider_model_id,
             "input": params.input,
         }
         if params.encoding_format is not None: