[core] Fix scan version for fallback branch

[ArnavBalyan]

Purpose

• During reads, when a fallback branch is configured, Paimon forwards the scan.version to the fallback reader.
• However this scan version can represent different commit timestamps across the main and fallback branch causing the wrong snapshot to be picked from fallback branch.
• Fix by mapping scan versions to the fallback snapshot using the main snapshot timestamp.

Tests

• UT