Ensure whitespace-only if branches still execute assign side effects

[Copilot]

IfStatement could short-circuit elsif/else branches marked IsWhitespaceOrCommentOnly, which skipped execution of non-output statements (notably assign). This caused branch-local state (e.g. media_list_to_show) to remain unset even when the branch condition matched.

• Execution semantics in IfStatement

  • Removed early returns in the false-condition path that skipped matched elsif/else branches when IsWhitespaceOrCommentOnly == true.
  • Matched branches now always run through their own WriteToAsync, preserving existing whitespace suppression while executing side effects.

• Regression coverage

  • Added focused tests in IfStatementTests for:
    • assign inside a whitespace-only else branch
    • assign inside a whitespace-only elsif branch
  • Both assert that context state is updated after branch execution.

• Behavioral impact

  • No change to rendered output behavior for whitespace/comment-only branches.
  • Restores expected side-effect behavior for stateful tags inside those branches.

{% if false %}
{% elsif true %}
  {% assign x = 'value' %}
{% else %}
  {% assign x = 'other' %}
{% endif %}
{{ x }}  {# now resolves to "value" instead of remaining undefined #}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Bug: Marking assign IsWhitespaceOrCommentOnly sometimes prevents the assignment from execution</issue_title>
<issue_description>This is caused by #909
At least when the assign is the only statement in the else branch of an if statement, the assignment will not be executed, as the whole else branch will be marked IsWhitespaceOrCommentOnly and therefore later the statements that actually produce output will render nothing, as there is no input evaluated. It might not be the only case with IsWhitespaceOrCommentOnly, but the one I've encountered

Example from Horizon's card-gallery.liquid

    {% liquid
      # Check if there are generic media that are not variant images

      # [ ]
      assign variant_images = product.images | where: 'attached_to_variant?', true | map: 'src'
      # 2
      assign generic_media_size = all_media.size | minus: variant_images.size
      # 0
      assign combined_listing_count = product.options_with_values | map: 'values' | map: 'product_url' | compact | size

      # false
      if combined_listing_count > 0 or has_applied_colour_filter 
        ...
      else
        # should be executed, but actually not, so it's null
        assign media_list_to_show = all_media 
      endif
      ...
     
      # empty
      capture slides
        for media in media_list_to_show
          capture slideshow_children
            render 'product-media', media: media, sizes: sizes, loading: loading, preview_image_only: true
          endcapture
          ...
          render 'slideshow-slide', slide_id: media.id, index: forloop.index, hidden: hidden, class: class, attributes: attributes, children: slideshow_children, media_fit: media_fit
      endcapture 
    %}
    <a>
      {% render 'slideshow', 
        # renders empty
        ref: 'slideshow',
        initial_slide: 0,
        slides: slides,
        slide_count: all_media.size,
        show_arrows: show_arrows,
        attributes: slideshow_attributes
      %}
    </a>
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Bug: Marking assign IsWhitespaceOrCommentOnly sometimes prevents the assignment from execution #921

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