injection_points: canceled or terminated waiters leak their wait slots

Hello hackers,

While stress-testing REPACK CONCURRENTLY on 19beta2 I saw a logical
decoding activation race, and I extended 051_effective_wal_level.pl
with a test that waits on an injection point and wakes it up later.
051 cancels two injection point waiters earlier in the script, and
injection_wait() never cleans up after a canceled waiter, the wakeup
never arrived and the test deadlocked.

I think we are missing an ENSURE_ERROR_CLEANUP block there. See
attached patch with a testcase reproducing the issue.

A wakeup racing against a canceled waiter with no other live waiter
now errors with "could not find injection point ... to wake up"
instead of silently bumping the leaked slot.

I also attached a separate version for pg19, as master has a
refactored version of injection_wait. All previous branches have the
19 version, it should be easy to backport to other branches.