fix: fetch with BODY.PEEK[] so a fetch never sets \Seen

RFC822 / BODY[] on a SELECTed mailbox set \Seen as a side effect (RFC
3501). with the UNSEEN query, one flow fetching and rejecting a message hid
it from another flow on the same mailbox. only mark_seen() flags a message
now; the icloud param is kept and is a no-op.

Signed-off-by: disqualifier <dev@disqualifier.me>

src/aiomail/client.py

@@ -229,10 +229,16 @@ class IMAPClient:
         return sorted(set(ids), reverse=True)
 
     async def fetch(self, email_id: int, *, icloud: bool = False) -> Optional[email.message.Message]:
-        """fetch and parse a full message by id"""
+        """fetch and parse a full message by id without setting \\Seen
+
+        uses BODY.PEEK[]: RFC822 / BODY[] on a SELECTed mailbox set \\Seen as a side effect
+        (RFC 3501), so a fetched-then-rejected message would vanish from every later UNSEEN
+        search. only mark_seen() flags a message. `icloud` is kept for compatibility and is
+        now a no-op (BODY.PEEK[] already works there)
+        """
         if not await self.ensure_connection():
             return None
-        part = "(BODY[])" if icloud else "(RFC822)"
+        part = "(BODY.PEEK[])"
         try:
             if self.use_uid:
                 result, data = await self._mail.uid("fetch", str(email_id), part)