fix: map all DM interval column subtypes

[skhe]

Summary

• Map all 13 DM interval column subtypes to their DPI type codes instead of returning 11 of them as ordinary strings.
• Verify day-second subtypes as datetime.timedelta, year-month subtypes with their interval description type, and positive/negative timedelta parameter writes.
• Record local UTF-8 and GB18030 results.

Validation

• Before fix: all 11 new subtype read cases failed on the official DM8 instance.
• Official ARM DM8 UTF-8: 183 passed, 2 deselected, 0 skipped.
• Official ARM DM8 GB18030: 183 passed, 2 deselected, 0 skipped.
• Go bridge: go test ./... passed.