Adds pre_vote_ep_err_handler_for_test (XCE_TESTING-guarded) and two
tests: PreVote post-connect failure reaches pre_vote_reply_dispatch's
tally (Req 4.3), and a round mixing that failure mode with a delivered
reply reaches its quorum decision via the fast all-replied path
without the full round timeout (Req 4.5). Real-vote coverage (Req 4.2)
was already proven by task 3.1's own test, cited not duplicated.
pre_vote_reply_dispatch has no mock hook seam, so both tests assert
directly on raft_state_t's tally fields, mirroring test_ha_prevote.c's
existing verification technique.
1061 -> 1063 tests, 0 failures.
Rongsong Shen
62db70d16d