Fix history sync on testnet & Simplifying history sync artificial checkpoint blocks

[ii-cruz]

What's in this pull request?

The extend history for syncing history nodes adds synthetic macro blocks if those are missing, so that it can recreate the FinalizeBatch inherents before pushing the history of an epoch. However, since we added the reward transactions to the body of the macro blocks, there is only one checkpoint block that can be empty - the first checkpoint block. This is because we pay rewards with a batch delay.
The pr #3758 did some hardening for the original assertion. However, it introduced a bug by rejecting the blocks when we have a gap of 2 batches. This shouldn't be the behaviour since we can have a valid code path for a gap, the case where we are in genesis block (batch 0) and batch 1 has no history, so the first item is on batch 2.

This PR:

• Fixes the introduced bug in 3758
• Removes and fixes extend_history_sync FIX MEs and outdated comments
• Simplifies the addition of the synthetic missing checkpoint blocks to only do that when it's for checkpoint 1.
• Adds unit tests for this edge cases

Pull request checklist

• All tests pass. The project builds and runs.
• I have resolved any merge conflicts.
• I have resolved all clippy and rustfmt warnings.

[Copilot]

Pull request overview

This PR simplifies extend_history_sync by tightening the “synthetic checkpoint insertion” logic to the first epoch scenario (where the first checkpoint macro block may have no history items due to delayed reward payouts). It also adds a regression test covering history sync for the first epoch.

Changes:

• Simplify history-sync checkpoint insertion logic in Blockchain::push_history_sync / extend_history_sync.
• Initialize prev_batch from the current chain head instead of assuming 0.
• Add a new test that syncs the first epoch via (a) first checkpoint then election, and (b) directly to the election block.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
blockchain/src/blockchain/history_sync.rs	Simplifies synthetic macro/checkpoint insertion while extending history during sync.
blockchain/tests/history_sync.rs	Adds coverage for syncing the first epoch (checkpoint + election, and direct election).

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