fix(golang): escape generated enum values (fix #4448)

[sjh9714]

Summary

Fixes PostgreSQL enum values containing Go string metacharacters being emitted into generated Go constants without string literal escaping.

What changed

• Updated the Go enum constant template to render enum values with Go string literal escaping (printf "%q").
• Added an end-to-end PostgreSQL stdlib fixture covering a backslash-n enum value and a double-quote/code-like enum value.

Why

The previous template wrapped raw enum values in quotes, so user\nadmin became a newline at runtime and values containing " could produce invalid generated Go code. The new template delegates literal formatting to Go's %q formatting.

Testing

• Red check: go test ./internal/endtoend -run 'TestReplay/base/enum_escaped_values/postgresql/stdlib' failed before the template change with a generated models.go diff missing escapes.
• go test ./internal/endtoend -run 'TestReplay/base/enum_escaped_values/postgresql/stdlib'
• go test ./internal/codegen/golang
• gofmt -l internal/endtoend/testdata/enum_escaped_values/postgresql/stdlib/go/db.go internal/endtoend/testdata/enum_escaped_values/postgresql/stdlib/go/models.go internal/endtoend/testdata/enum_escaped_values/postgresql/stdlib/go/query.sql.go
• go test ./...
• go test --tags=examples -timeout 20m ./...

Fixes #4448