test: Cover signature_slot tiebreaker in is_better_update

[bshastry]

The is_better_update function has a final tiebreaker that compares signature_slots when all other fields (participation, finality, etc.) are equal. This branch was previously uncovered because test updates always had equal slots.

This change adds a test case with a higher signature_slot to an otherwise identical update, ensuring we test the final return statement where earlier signature_slots are preferred.

The change is minimal and leverages existing test infrastructure by adding one additional update to create_test_update quality-ordered list.

[jtraglia]

This looks reasonable to me, but I'm not super familiar with light client stuff either. I would appreciate @etan-status' opinion on this change before merging.

The relevant tiebreaker (the final return statement). I might suggest adding a "Tiebreaker 3: ..." comment too.

consensus-specs/specs/altair/light-client/sync-protocol.md

Lines 272 to 275 in 007e2c9

	# Tiebreaker 2: Prefer older data (fewer changes to best)
	if new_update.attested_header.beacon.slot != old_update.attested_header.beacon.slot:
	return new_update.attested_header.beacon.slot < old_update.attested_header.beacon.slot
	return new_update.signature_slot < old_update.signature_slot

[etan-status]

Thanks for catching this, having the additional test coverage is useful. 🔥

[bshastry]

This looks reasonable to me, but I'm not super familiar with light client stuff either. I would appreciate @etan-status' opinion on this change before merging.

The relevant tiebreaker (the final return statement). I might suggest adding a "Tiebreaker 3: ..." comment too.

consensus-specs/specs/altair/light-client/sync-protocol.md

Lines 272 to 275 in 007e2c9

	# Tiebreaker 2: Prefer older data (fewer changes to best)
	if new_update.attested_header.beacon.slot != old_update.attested_header.beacon.slot:
	return new_update.attested_header.beacon.slot < old_update.attested_header.beacon.slot
	return new_update.signature_slot < old_update.signature_slot

Done! Pushed an update that may need a reapproval. Thank you 🙏

[bshastry]

Thank you for your reviews @jtraglia and @etan-status

I have addressed your review comments