Prefer close backfill points (absolute distance)

This isn't fixing any particular issue. It's just a follow-up I thought about after merging #19611 since we're now also dealing with backfill points in the nearby range ahead of the current_depth. And it's possible that the previous sort could bias to all nearby backfill points ahead of the current_depth that don't extend into the visible window of events we're paginating through.

Todo

• Quick sanity check that the sort works with the tuple

Dev notes

Sort sanity check:

current_depth = 35
actual_sorted_list = sorted(
	[10, 20, 25, 30, 35, 40, 50],
	key=lambda depth: (
	    # Prefer backfill points that are closer to the `current_depth`
	    # (absolute distance)
	    abs(current_depth - depth),
	    # For the tie-break, we care about events that are actually in the past
	    # as they're more likely to reveal history that we can return (something
	    # absolutely in the past is better than something can potentially extend
	    # into the past).
	    #
	    # This sorts ascending so 0 sorts before 1
	    0 if current_depth >= depth else 1,
	),
)

expected_sorted_list = [35, 30, 40, 25, 20, 50, 10]

assert actual_sorted_list == expected_sorted_list, f"Expected actual list ({actual_sorted_list}) to be sorted like the expected list ({expected_sorted_list})"
print("Success")

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