[Bug]: Unable to use endpoint switching when original endpoint is deleted

[RedwindA]

What happened?

When attempting to use the "Enable switching Endpoints mid-conversation" feature in a conversation where the original endpoint has been deleted, the feature does not work properly. This prevents users from continuing conversations with a different endpoint when the original one is no longer available.

Steps to Reproduce

1. Start a conversation using an endpoint (e.g. OpenAI)
2. Delete that endpoint in librechat.yaml
3. Try to use "Enable switching Endpoints mid-conversation" feature in that existing conversation
4. The feature fails to work. User will be redirected to /c/new path

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