allow rollback within _prepare_impl on twophase prepare failure

allow rollback within _prepare_impl on twophase prepare failure

When tpc_prepare() raised during SessionTransaction._prepare_impl(),
the error handler's call to self.rollback() was blocked by the
@declare_states decorator, which had set _next_state to
CHANGE_IN_PROGRESS. This caused IllegalStateChangeError to be raised
instead of the original database exception, masking the real error
and preventing proper cleanup.

Used _expect_state(SessionTransactionState.CLOSED) to temporarily
allow the rollback state transition, matching the existing pattern
used in commit() for the close() call.

Fixes: #13356
Change-Id: Ie8212d5b6f8515340cf9d83c56dcbfa5a7415812
(cherry picked from commit 30d75f9a30beab059b6ce93d2c200fd8944d0eb9)