fix(validation): respect settings-bound provider and correct multimodal heuristic

backend/src/services/llm_prompt_templates.py

@@ -161,13 +161,9 @@ def is_multimodal_model(model_name: str, provider_type: Optional[str] = None) ->
         "internvl",
         "qwen-vl",
         "qwen2-vl",
-        "stepfun/step-3.5",
     )
     if any(marker in token for marker in multimodal_markers):
         return True
-    # OpenRouter model ids are heterogeneous; keep permissive path for known StepFun family.
-    if provider == "openrouter" and token.startswith("stepfun/step-3.5"):
-        return True
     return False
 # [/DEF:is_multimodal_model:Function]