Page sandbox doesn't expand YAML-level Jinja

Can be observed with `homework-problems/inteq_bvp/medium_inteq_bvp_code_second_order.yml` in the CS450 repo. This does

```yaml
{% from "yaml-macros.jinja" import indented_include %}
[...]
setup_code: |J

{{ indented_include("homework-problems/inteq_bvp/bvp_test_problems.py", 4) }}
{{ indented_include("homework-problems/inteq_bvp/order_est.py", 4) }}
```

Reproduce the issue with homework-problems/inteq_bvp/medium_inteq_bvp_code_second_order.yml and compare the YAML-level Jinja with the referenced bvp_test_problems.py and order_est.py files. Trace the page sandbox's YAML processing to determine why the indented_include calls are not expanded; done means the shown setup_code includes the referenced code as intended.

Written by the indexing model from the issue text.