refactor(ast): add named DropBehavior constants

The pg_query DropBehavior enum currently lives as a bare `type DropBehavior uint` —
callers that compare against the wire values have to write magic numbers
(`stmt.Behavior == 2` for CASCADE). Add named constants matching pganalyze/pg_query_go's
DropBehavior enum so usage sites can read `stmt.Behavior == ast.DropBehaviorCascade`
instead, and survive a future pg_query enum reshuffle without silently miscompiling.

No behavior change — just constants. Existing code that compares to integer literals
continues to work; this is purely additive.

Follow-up to the review on #4419 (which currently uses `Behavior == 2` literal):
happy to apply the same rename in that PR's diff once it lands, or in a separate
sweep PR depending on what the maintainers prefer.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Nguyễn Phúc Lương

internal/sql/ast/drop_behavior.go

@@ -1,7 +1,20 @@
 package ast
 
+// DropBehavior captures whether a Postgres DROP statement was qualified with
+// RESTRICT or CASCADE. The numeric values mirror pganalyze/pg_query_go's
+// DropBehavior enum:
+//
+//	DropBehavior_UNDEFINED = 0  (no behavior word supplied — same as RESTRICT in PG)
+//	DROP_RESTRICT          = 1
+//	DROP_CASCADE           = 2
 type DropBehavior uint
 
+const (
+	DropBehaviorUndefined DropBehavior = 0
+	DropBehaviorRestrict  DropBehavior = 1
+	DropBehaviorCascade   DropBehavior = 2
+)
+
 func (n *DropBehavior) Pos() int {
 	return 0
 }