Reset IMAP state to AUTH after unselect_folder() (fixes #639)

[apoorvdarshan]

Summary

unselect_folder() leaves the connection in the SELECTED state, so a subsequent enable() raises IllegalStateError (issue #639):

imap.select_folder("INBOX")
imap.unselect_folder()
imap.enable("UTF8=ACCEPT")   # -> IllegalStateError: ENABLE command illegal in state SELECTED

The root cause is that unselect_folder() calls self._imap._simple_command("UNSELECT") directly and never resets self._imap.state back to "AUTH". In contrast, imaplib.IMAP4.unselect() sets self.state = "AUTH" when the command succeeds, and close_folder() (which goes through imaplib's close) works correctly for the same reason.

Fix

After a successful UNSELECT (typ == "OK"), set self._imap.state = "AUTH", mirroring imaplib.IMAP4.unselect(). This returns the connection to the authenticated state so enable() and other AUTH-state commands are legal again. The change is localized to unselect_folder() in imapclient/imapclient.py.

Tests

Added test_unselect_resets_state_to_auth in tests/test_imapclient.py (alongside the existing test_unselect). It sets the mock state to "SELECTED", calls unselect_folder(), and asserts the state is reset to "AUTH". The test fails on the pristine code and passes with the fix. Full suite: 268 tests pass. black, isort, and flake8 are clean on the changed files.

Disclosure: prepared with AI assistance; reviewed and verified locally.