Introduced `FetchEmailByUidQuery` to fetch a single email by its
UID from an IMAP mailbox. The query includes properties for
email account, UID, folder, and attachment inclusion, with a
fluent method `WithUid(long uid)` to set the UID.
Added `FetchEmailByUidQueryHandler` to process the query,
leveraging `ISender` for account retrieval and `IImapEmailService`
for email fetching. Included error handling for missing accounts
and misconfigured IMAP settings.
Implemented `FetchEmailByUidQueryValidator` to validate the
query, ensuring the `Account` is not null, `Uid` is set and
greater than 0, and `Folder` is not empty.
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