Remove post-persist ambiguity and have events match emitted state

[brandur]

This one's aimed at fixing [1] in which for some sequences in workers,
we'd emit a surprising event based on the state that was actually
persisted to the database. This contract was also not stable, and
changed subtly with the introduction of #1219.

From [1], this is best illustrated by a short example where we error
after invoking JobCompleteTx:

func (w *Worker) Work(ctx context.Context, job *river.Job[Args]) error {
      tx, _ := w.dbPool.Begin(ctx)
      defer tx.Rollback(ctx)

      river.JobCompleteTx[*riverpgxv5.Driver](ctx, tx, job) // row -> completed
      tx.Commit(ctx)

      return errors.New("boom") // executor reports an error, but its UPDATE is an IfRunning no-op
}

This used to emit a job_completed, but has changed in master to emit
a job_failed.

Here, we try to correct the emitted event and officially standardize it:

Scenario	Persisted state	Previous event	New event
JobCompleteTx commits, then worker returns an error	completed	job_failed	job_completed
Remote cancellation, then worker requests a retry	cancelled	job_failed	job_cancelled
Remote cancellation, then worker snoozes	cancelled	job_snoozed	job_cancelled

Unambiguous persisted states always take precedence, though we retain
reason (added in #1219) to distinguish possible states of available,
which may be (1) an immediate retry after failure, (2) a short snooze,
or (3) an interruption caused by client shutdown.

[1] #1290 (comment)