ImportSingleMessageAsync now fetches the message + connection through the
provider-neutral seam (Email.GetMessageAsync/GetConnectionAsync), mapping the
neutral ExternalAttachmentId onto CorrespondenceAttachmentMetadata. The
controller's import path no longer touches Gmail directly.
OAuth lifecycle, the rich connection-status DTO, and Gmail candidate ranking
stay on IGmailOAuthService until a second provider (Microsoft/IMAP) forces the
contract shape. 135/135 green.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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