fix TiDB explain format conversion to MySQL compatible one

- Map all TiDB operator types to MySQL-compatible access types
- Add support for additional TiDB operators:
  - Batch_Point_Get → const (multiple point lookups)
  - TableRowIDScan → ref (rowid lookup)
  - TableScan/IndexScan → ALL/index (generic scans)
  - IndexLookUp → ref (non-unique index lookup)
- Improve mapping accuracy for better MySQL compatibility

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: takaidohigasi