chore: update return type to Optional[str]

Updated the return type of the methods `get_provider_model_id` and
`get_llama_model` in the `ModelRegistryHelper` class to `Optional[str]`
to indicate that they may return a string or None when no match is
found. This change improves the clarity of the methods' behavior and
supports better type safety.

Signed-off-by: Sébastien Han <seb@redhat.com>

llama_stack/providers/utils/inference/model_registry.py

@@ -57,13 +57,13 @@ def __init__(self, model_aliases: List[ModelAlias]):
             self.alias_to_provider_id_map[alias_obj.llama_model] = alias_obj.provider_model_id
             self.provider_id_to_llama_model_map[alias_obj.provider_model_id] = alias_obj.llama_model
 
-    def get_provider_model_id(self, identifier: str) -> str:
+    def get_provider_model_id(self, identifier: str) -> Optional[str]:
         if identifier in self.alias_to_provider_id_map:
             return self.alias_to_provider_id_map[identifier]
         else:
             return None
 
-    def get_llama_model(self, provider_model_id: str) -> str:
+    def get_llama_model(self, provider_model_id: str) -> Optional[str]:
         if provider_model_id in self.provider_id_to_llama_model_map:
             return self.provider_id_to_llama_model_map[provider_model_id]
         else: