feat(sql): implement DISTINCT support

[aviralgarg05]

Summary

Implement DISTINCT keyword support in SELECT queries.

Why

DISTINCT is a core SQL feature and listed in the roadmap, but currently unsupported.

Tasks

• Extend SQL types/parser to represent DISTINCT
• Update executor path to deduplicate projected rows correctly
• Add parser, unit, and integration tests

Acceptance Criteria

• SELECT DISTINCT column FROM table works as expected
• Deduplication happens after projection
• Tests cover text, numeric, and multi-row duplicate scenarios