Add pending:true support for section helper in Go implementation

## Summary

The Go implementation of Dotprompt is missing the `pending: true` option for the section helper.

## Location

- `go/dotprompt/helper.go:37`: `// TODO: Add pending: true for section helper`

## Acceptance Criteria

- [ ] Add `pending` option to section helper in Go
- [ ] Ensure parity with JS implementation
- [ ] Add tests for pending section behavior