fix(s3wal): improve next() logic to handle pre-fetched records

[superhx]

• [Bug] the nextRecord is not cleaned up when directly calling #next without calling #hasNext
• The bug won't happen in production because each #next is called after #hasNext

[Copilot]

Pull request overview

This PR fixes a bug in the RecoverIterator.next() method where the nextRecord field was not properly cleared in all code paths, potentially causing the same record to be returned multiple times.

Key Changes:

• Consolidated the two separate conditional checks into a single if-else statement with a combined condition
• Added explanatory comments describing when nextRecord is already available versus when hasNext() populates it

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