fix(archiver): handle duplicate checkpoint from L1 reorg (backport #22252)

[AztecBot]

Summary

Backport of #22252 to v4.

Fixes a mainnet issue where an L1 reorg moved a checkpoint to a different L1 block, causing the archiver to re-discover it and crash with InitialCheckpointNumberNotSequentialError in an infinite loop.

When addCheckpoints receives a checkpoint that's already stored, it now:

• Accepts it if the archive root matches (same content, just different L1 block)
• Updates the L1 metadata (block number, timestamp, hash) and attestations
• Throws if the archive root doesn't match (content mismatch — genuine conflict)

Conflict Resolution

Cherry-pick had one conflict in block_store.ts:

• v4 uses inline code for extracting previous checkpoint block data; next refactored this into a getPreviousCheckpointBlock() helper. Kept v4's inline approach but updated to use checkpoints[0] reference (correct after potential skip of already-stored checkpoints).
• Only ported skipOrUpdateAlreadyStoredCheckpoints method (the fix). Dropped getPreviousCheckpointBlock and validateCheckpointBlocks refactors that exist on next but not v4.

Test Results

• 176 passed in kv_archiver_store.test.ts
• 33 passed in archiver-sync.test.ts (includes new handles L1 reorg that moves a checkpoint to a later L1 block test)
• Build, format pass

ClaudeBox log: https://claudebox.work/s/21a4c138364df1ef?run=1