Refactor SMTP handling to avoid OAuth usage

[wreckage0907]

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This pull request updates the logic for determining when to use OAuth authentication in the sendmail_config method of email_account.py. Instead of relying on the auth_method field, it now checks if both custom_outgoing_server_username and custom_outgoing_server_password are not set.

Authentication logic update:

• Changed the calculation of use_oauth to be true when both custom_outgoing_server_username and custom_outgoing_server_password are not set, instead of checking if auth_method is "OAuth". (frappe_email_send_override/overrides/email_account.py)

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