Support to read chain data split

[Weixin-Xu]

Purpose

Linked issue: close #385

Support deserializing and reading ChainDataSplit.

This change adds ChainDataSplit handling on top of DataSplit deserialization, including:

• reading ChainDataSplit tail metadata after DataSplit bytes
• supporting per-file bucket path mapping when building data file paths
• preserving normal DataSplit / FallbackDataSplit behavior
• supporting the observed version 7 DataSplit metadata layout needed by chain split bytes

Tests

• Added ChainDataSplitTest
  • deserialize ChainDataSplit tail
  • deserialize version 7 split with original bucket path and chain tail
  • verify per-file bucket path resolution
  • verify external path is preserved
  • verify malformed ChainDataSplit tail returns contextual error

API and Format

Documentation

No user-facing documentation update.

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