🛠️ fix: Custom Endpoint issues, Improve SSE Response Handling (#1510)

* fix(custom): prevent presets using removed custom endpoints from causing frontend errors

* refactor(abortMiddleware): send 204 status when abortController is not found/active, set expected header `application/json` when not set

* fix(useSSE): general improvements:
- Add endpointType to fetch URL in useSSE hook
- use EndpointURLs enum
- handle 204 response by setting `data` to initiated response
- add better error handling UX, make clear when there is an explicit error

client/src/components/Chat/Menus/Endpoints/MenuItem.tsx

@@ -82,7 +82,7 @@ const MenuItem: FC<MenuItemProps> = ({
         <div className="flex grow items-center justify-between gap-2">
           <div>
             <div className="flex items-center gap-2">
-              {
+              {Icon && (
                 <Icon
                   size={18}
                   endpoint={endpoint}
@@ -90,7 +90,7 @@ const MenuItem: FC<MenuItemProps> = ({
                   className="icon-md shrink-0 dark:text-white"
                   iconURL={getEndpointField(endpointsConfig, endpoint, 'iconURL')}
                 />
-              }
+              )}
               <div>
                 {title}
                 <div className="text-token-text-tertiary">{description}</div>