test: add regression test for nested optionals (Issue #49)

[rockaxorb13]

Fixes #49

Description

This PR adds a regression test case using & improving the reproduction files provided in Issue #49.

Changes

• Added test/templates/good/optional-nested containing the reproduction model, data, and template.
• Added a custom "Negative Test" case to template.md to verify top-level optionals alongside nested ones.
• Updated snapshots to reflect the correct resolution of nested optional variables (e.g., {{uri}} resolving to the URL instead of being empty).

Verification

The test already passes on the current main branch, confirming that the logic for switching scope in OptionalBlock is working correctly.

[mttrbrts]

@niallroche, does this match your expectations too?

[niallroche]

thanks @rockaxorb13 I can see it is resolving now, I do notice that there are newlines placed between the variables that does create layout issues, I am not sure if this is a separate issue or a side effect of any related work?

[rockaxorb13]

Thanks, @niallroche!

I reviewed the snapshot output (optional-nested), and you are right—there are Softbreak nodes and nested Paragraph structures creating vertical whitespace between the variables.

However, I believe this is pre-existing behavior in how the engine generates CommonMark for nested blocks. My PR didn't modify the AST generation logic; I just added the test case to check and ensure that the nesting logic resolves correctly (which it now does).

The test captures the current reality of the engine. I'm happy to open a separate issue to track 'whitespace/layout cleanup' for the engine so we can merge this regression test first.