fix: catch missing output with reconnecting PTY (#9094)

I forgot that waiting on the cond releases the lock so it was possible
to get pty output after writing the buffer but before adding the pty to
the map.  To fix, add the pty to the map while under the same lock where
we read from the buffer.

The rest does not need to be behind the lock so I moved it out of
doAttach, and that also means we no longer need
waitForStateOrContextLocked.

Also, this can hit a logger error saying the attach failed which fails
the tests however it is not that the attach failed, just that the
process already ran and exited, so when the process exits do not
set an error, instead for now assume this is an expected close.

agent/agent.go

@@ -1134,7 +1134,7 @@ func (a *agent) handleReconnectingPTY(ctx context.Context, logger slog.Logger, m
 			rpty.Wait()
 			a.reconnectingPTYs.Delete(msg.ID)
 		}); err != nil {
-			rpty.Close(err.Error())
+			rpty.Close(err)
 			return xerrors.Errorf("start routine: %w", err)
 		}