fix: ensure show_filters_icon is always returned in API responses

Set default value for show_filters_icon in LinkBase schema to ensure the field is always included in API responses, even when response_model_exclude_unset=True is used.

gns3server/schemas/controller/links.py

@@ -62,7 +62,7 @@ class LinkBase(BaseModel):
     suspend: Optional[bool] = None
     link_style: Optional[LinkStyle] = None
     filters: Optional[dict] = None
-    show_filters_icon: Optional[bool] = None
+    show_filters_icon: Optional[bool] = Field(default=True, description="Show filters icon in Web UI")
 
 
 class LinkCreate(LinkBase):