Fix types to let OpenRouterProvider be created with only http_client (#3308)

Co-authored-by: Douwe Maan <[email protected]>

Author: xcpky

pydantic_ai_slim/pydantic_ai/providers/openrouter.py

@@ -81,6 +81,9 @@ def __init__(self, *, api_key: str) -> None: ...
     @overload
     def __init__(self, *, api_key: str, http_client: httpx.AsyncClient) -> None: ...
 
+    @overload
+    def __init__(self, *, http_client: httpx.AsyncClient) -> None: ...
+
     @overload
     def __init__(self, *, openai_client: AsyncOpenAI | None = None) -> None: ...