refactor(ast): add named DropBehavior constants

[luongs3]

Adds named DropBehavior constants to internal/sql/ast/drop_behavior.go. No behavior change — purely additive.

Why

internal/sql/ast currently has:

type DropBehavior uint

func (n *DropBehavior) Pos() int {
	return 0
}

The numeric values come from pganalyze/pg_query_go's DropBehavior enum, but they are nowhere named on the sqlc side. Callers comparing against the wire value have to write stmt.Behavior == 2 for CASCADE — a magic number that's invisible to grep and silently miscompiles if pg_query reshuffles the enum.

This PR adds:

const (
	DropBehaviorUndefined DropBehavior = 0
	DropBehaviorRestrict  DropBehavior = 1
	DropBehaviorCascade   DropBehavior = 2
)

Comments at the top of the file document where the values come from so future contributors don't have to chase the pg_query source.

What does NOT change

• No use-site is updated in this PR. Existing comparisons against integer literals continue to compile.
• No behavior or AST shape changes — only constants.
• No new tests required: the constants are typed and validated by the compiler.

Context

This is the follow-up I offered in a review note on #4419. That PR uses stmt.Behavior == 2 in internal/sql/catalog/table.go. Once the constants land, refactoring those use-sites to stmt.Behavior == ast.DropBehaviorCascade is a 1-line change — happy to bundle into #4419 (if maintainers prefer one PR) or send as a separate sweep PR after #4419 merges. Both work; I have no preference.