fix: imap idle writter with nil connection

[plusiv]

There's an intermittent panic that occurs because an IDLE writer was registered with a nil textproto.Conn; when SMTP appends a message, Folder.Append notifies IDLE writers and WriteNumMessages dereferences w.tpc, causing a nil-pointer panic. It only fails sometimes because the notification depends on timing, if no IDLE session is active or the writer has already been removed, no call happens. The fix is to construct idleWriter with the active connection (newIdleWriter(tpc *textproto.Conn)) so updates write through the IMAP connection.

This pull request refactors the handling of the idleWriter in the IMAP IDLE implementation to improve code clarity and correctness. The main updates include introducing a constructor for idleWriter, updating its instantiation, and fixing the format of the EXPUNGE response.

Refactoring and correctness improvements:

• Added a newIdleWriter constructor function for idleWriter to encapsulate initialization logic and replaced the previous direct instantiation with this constructor in the handleIdle method. [1] [2]
• Corrected the IMAP EXPUNGE response format in WriteExpunge to include the required * prefix, ensuring compliance with the IMAP protocol.

[marle3003]

Thank you for your contribution and for submitting this PR