Email length error doesn't appear if ESP is "manual"

[dkoo]

#764 added an error message to the block editor when the email content length approaches 100kb. This error message doesn't appear when the active ESP is set to "Manual / Other".

1. On a fresh site, in Newsletters > Settings, set the Service Provider to "Mailchimp", "Constant Contact", or "Campaign Monitor", ensuring you have credentials entered for the selected ESP.
2. Create or edit a newsletter. Add more than 100kb of text. Observe the "Email content is too long and may get clipped by email clients" warning in the editor.
3. In Newsletters > Settings, set the Service Provider to "Manual / Other".
4. Edit the long newsletter and observe that the warning is no longer shown.

Expected: The warning should be shown regardless of the active ESP.