core: do not connect to a group whose member is already connected

Connecting to a prepared group whose host or relay member already had a
connection created a second one for the same member -

  UNIQUE constraint failed: connections.group_member_id

reachable by connecting again from a second view of the same group. joinContact
advances the connection past ConnPrepared only after the agent call returns, so
a join that failed is resumed by the ConnPrepared branch above and reaching this
one means the join already succeeded; there is no case where connecting again is
what the caller wants. Replaces a connect' call the code already questioned.

The two tests reach this error only because owner members are now re-used, so
they cover both halves: without either change they fail on a constraint instead.

src/Simplex/Chat/Library/Commands.hs

@@ -3771,7 +3771,10 @@ processChatCommand cxt nm = \case
           Nothing -> connect' groupLinkId Nothing (Just $ Just gInfo)
           Just conn@Connection {connStatus, xContactId} -> case connStatus of
             ConnPrepared -> joinPreparedConn' xContactId conn (Just $ Just gInfo)
-            _ -> connect' groupLinkId xContactId (Just $ Just gInfo) -- why not "already connected" for host member?
+            -- the connection is only advanced past ConnPrepared once the join succeeded (see joinContact),
+            -- so retrying a join that failed reuses it above, and reaching here means it already joined -
+            -- connecting again would create a second connection for the same member.
+            _ -> throwCmdError "group is already being joined"
         Nothing ->
           withFastStore' (\db -> getConnReqContactXContactId db cxt user cReqHash1 cReqHash2) >>= \case
             Right ct@Contact {activeConn} -> case groupLinkId of