gh-88574: Do not swallow the line after a terminating literal in imaplib (GH-153317)

gh-88574: Do not swallow the line after a terminating literal in imaplib (GH-153317)

GH-152751 skipped a spurious blank line after a literal unconditionally,
corrupting a response that ends with a literal (such as a mailbox name
returned by LIST): its empty trailer was mistaken for the blank and the
following line was swallowed.

The blank is now skipped only inside an unclosed parenthesis.  After a
literal that ends the response it instead arrives before the next
response and is skipped there.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>