fix: pg-meta returning duplicate columns

[soedirgo]

What kind of change does this PR introduce?

Bug fix

What is the current behavior?

If a column has multiple check constraints, pg-meta returns duplicate entries on GET /columns and generated types.

What is the new behavior?

Fix the bug

Additional context

Fixes supabase/postgres-meta#590

[coveralls]

Pull Request Test Coverage Report for Build 5898012083

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.04%) to 56.87%

---

Files with Coverage Reduction	New Missed Lines	%
internal/gen/keys/keys.go	5	12.31%

Totals
Change from base Build 5898005518:	-0.04%
Covered Lines:	4851
Relevant Lines:	8530

---

💛 - Coveralls