fix: improve message formatting and set default activation for providers

- In PerplexityLabs.py, added logic to filter consecutive assistant messages and update message array accordingly
- Modified PerplexityLabs.py to change "messages" field to use the new formatted message list
- Adjusted error handling in PerplexityLabs.py to include a newline in error messages
- Import os in BlackForestLabs_Flux1KontextDev.py and replace media filename assignment with basename if media is None
- In Groq.py, set "active_by_default" to True for the provider
- In OpenRouter.py, added "active_by_default" as True
- In Together.py, set "active_by_default" to True
- In HuggingFaceInference.py, set "working" to False
- In models.py, changed default_model to "openai/gpt-oss-120b" instead of previous value
- In backend_api.py, added a null check in jsonify_provider_models to return 404 if response is None, and simplified get_provider_models call

g4f/gui/server/backend_api.py

@@ -122,6 +122,8 @@ class Backend_Api(Api):
         def jsonify_provider_models(**kwargs):
             try:
                 response = self.get_provider_models(**kwargs)
+                if response is None:
+                    return jsonify({"error": {"message": "Provider not found"}}), 404
             except MissingAuthError as e:
                 return jsonify({"error": {"message": f"{type(e).__name__}: {e}"}}), 401
             except Exception as e:
@@ -596,10 +598,7 @@ class Backend_Api(Api):
         api_key = request.headers.get("x_api_key")
         api_base = request.headers.get("x_api_base")
         ignored = request.headers.get("x_ignored", "").split()
-        models = super().get_provider_models(provider, api_key, api_base, ignored)
-        if models is None:
-            return "Provider not found", 404
-        return models
+        return super().get_provider_models(provider, api_key, api_base, ignored)
 
     def _format_json(self, response_type: str, content = None, **kwargs) -> str:
         """