fix(l1): skip fork-choice update if snap-syncing

[MegaRedHand]

Motivation

We recently found some errors due to payloads being marked as invalid when FCU is triggered during a snap-sync.

Description

This PR uncomments an early return that handles these cases.

Closes #5547

[github-actions]

Lines of code report

Total lines added: 12
Total lines removed: 0
Total lines changed: 12

Detailed view

+----------------------------------------------------+-------+------+
| File                                               | Lines | Diff |
+----------------------------------------------------+-------+------+
| ethrex/crates/networking/rpc/engine/fork_choice.rs | 371   | +12  |
+----------------------------------------------------+-------+------+

[Copilot]

Pull request overview

This PR fixes an issue where payloads were incorrectly marked as invalid during snap-sync by uncommenting a guard clause that handles fork-choice updates during snap-sync operations.

Key changes:

• Uncommented the snap-sync check in handle_forkchoice function that skips fork-choice processing during snap-sync
• Added import for SyncMode from ethrex_p2p::sync
• Fixed reference from context.syncer to syncer to match the function's local variable

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