db: TestIteratorErrors flakes

[jbowens]

https://github.com/cockroachdb/pebble/actions/runs/20278725717/job/58234611105?pr=5662

--- FAIL: TestIteratorErrors (0.83s)
    external_test.go:40: Using seed 1765910197256341000

panic: pebble: *sstable.singleLevelIterator[github.com/cockroachdb/pebble/sstable/colblk.IndexIter,*github.com/cockroachdb/pebble/sstable/colblk.IndexIter,github.com/cockroachdb/pebble/sstable/colblk.DataBlockIter,*github.com/cockroachdb/pebble/sstable/colblk.DataBlockIter] point iterator returned non-nil key "ltdema@18446744073709551614#inf,SYNTHETIC" while iter has error [recovered, repanicked]

goroutine 152198 [running]:
testing.tRunner.func1.2({0x101203d40, 0x14015656cf0})
	/Users/runner/hostedtoolcache/go/1.25.5/arm64/src/testing/testing.go:1872 +0x190
testing.tRunner.func1()
	/Users/runner/hostedtoolcache/go/1.25.5/arm64/src/testing/testing.go:1875 +0x31c
panic({0x101203d40?, 0x14015656cf0?})
	/Users/runner/hostedtoolcache/go/1.25.5/arm64/src/runtime/panic.go:783 +0x120
github.com/cockroachdb/pebble_test.TestIteratorErrors.func2()
	/Users/runner/work/pebble/pebble/external_test.go:101 +0x90
panic({0x101203d40?, 0x14015656cf0?})
	/Users/runner/hostedtoolcache/go/1.25.5/arm64/src/runtime/panic.go:783 +0x120
github.com/cockroachdb/pebble/internal/base.AssertionFailedf(...)
	/Users/runner/work/pebble/pebble/internal/base/error.go:43
github.com/cockroachdb/pebble/internal/keyspan.(*InterleavingIter).savePoint(0x14014d31b30, 0x140001bd500)
	/Users/runner/work/pebble/pebble/internal/keyspan/interleaving_iter.go:1110 +0x104
github.com/cockroachdb/pebble/internal/keyspan.(*InterleavingIter).SeekPrefixGE(0x14014d31b30, {0x140155feba0, 0x6, 0x18}, {0x148b09d39, 0x8, 0x8}, 0x6?)
	/Users/runner/work/pebble/pebble/internal/keyspan/interleaving_iter.go:323 +0xd8
github.com/cockroachdb/pebble.(*levelIter).SeekPrefixGE(0x14014d31920, {0x140155feba0, 0x6, 0x18}, {0x148b09d39, 0x8, 0x8}, 0x0)
	/Users/runner/work/pebble/pebble/level_iter.go:720 +0x260
github.com/cockroachdb/pebble.(*mergingIter).seekGE(0x14014d2f7c0, {0x140147708d0?, 0x5?, 0x0?}, 0x1400e40f438?, 0x50?)
	/Users/runner/work/pebble/pebble/merging_iter.go:995 +0x248
github.com/cockroachdb/pebble.(*mergingIter).SeekPrefixGEStrict(0x14014d2f7c0, {0x140155feba0, 0x6, 0x18}, {0x140147708d0, 0x8, 0x10}, 0x0)
	/Users/runner/work/pebble/pebble/merging_iter.go:1082 +0x164
github.com/cockroachdb/pebble.(*mergingIter).SeekPrefixGE(0x140147708d0?, {0x140155feba0?, 0x10?, 0x140151bce47?}, {0x140147708d0?, 0x11?, 0x5500?}, 0x70?)
	/Users/runner/work/pebble/pebble/merging_iter.go:1067 +0x24
github.com/cockroachdb/pebble.(*Iterator).SeekPrefixGE(0x14014d2f190, {0x140147708d0, 0x8, 0x10})
	/Users/runner/work/pebble/pebble/iterator.go:1555 +0x5a0
github.com/cockroachdb/pebble/metamorphic.(*retryableIter).SeekPrefixGE.func1()

Jira issue: PEBBLE-1304