fix: cache slot in check_block_relevancy to prevent TOCTOU

[radik878]

Why

check_block_relevancy called chain.slot()? twice — once for the comparison and once inside the FutureSlot error. Since chain.slot()

lighthouse/beacon_node/beacon_chain/src/block_verification.rs

Lines 1247 to 1249 in edba56b

	pub fn slot(&self) -> Slot {
	self.block.slot()
	}

reads the current time from the slot clock, the slot can advance between the two calls at a slot boundary. This causes present_slot in the error to be higher than block_slot, which is logically contradictory for a "future slot" error and can mislead logging and peer scoring.

Every other FutureSlot check in the codebase (gossip blocks, blobs, data columns, attestations, sync committees) already caches the slot in a local variable. This was the only place that didn't.

What

Store chain.slot()? in a local variable and reuse it for both the comparison and the error construction, consistent with the rest of
the codebase.

[cla-assistant]

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[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[michaelsproul]

#Sounds reasonable, can you please rebase on unstable?

[michaelsproul]

I've changed the base branch on github but you will need to push a rebased commit for CI to pass.

Please also sign the CLA