Adds an account-level setting `keep_pending_on_bot_failure` to control
whether conversations should move from pending to open when agent bot
webhooks fail.
Some users experience occasional message drops and don't want
conversations to automatically reopen due to transient bot failures.
This setting gives accounts control over that behavior. This is a
temporary setting which will be removed in future once a proper fix for
it is done, so it is not added in the UI.
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