[format] Validate blob file footer and index metadata

[Stephen0421]

Purpose

Harden BLOB file metadata parsing against corrupt or truncated footer and index data.

This change:

• Validates the minimum footer size and index boundaries before seeking or allocating memory.
• Rejects malformed compressed indexes with a descriptive IOException.
• Validates record-length sentinels, minimum record length, and the total indexed data size.
• Validates selection cardinality and selected record positions.
• Preserves the existing lazy payload-reading behavior.

Test

• Added BlobFileMetaTest covering:
  • Truncated files.
  • Invalid index lengths and malformed indexes.
  • Unsupported versions.
  • Invalid record lengths and mismatched data-region sizes.
  • NULL and placeholder sentinels.
  • Empty, non-contiguous, and out-of-range selections.

[JingsongLi]

+1