fix: preserve text content with bindings after void elements

[janechu]

Description

Fix text nodes and {{bindings}} being silently dropped when they follow HTML void elements like <br>, <hr>, <img>, etc.

Tree-sitter HTML parses void elements with following text as children of the void element node. For example, <br>Part B: {{value2}} is parsed as a single element node where Part B: {{value2}} is a text child of <br>. The parser previously skipped content processing for void elements (since they have no end tag), which caused any text nodes and bindings that tree-sitter attached as children to be silently dropped.

The fix adds process_content_children() for void elements, matching the behavior already used for elements with end tags.

Before:

Input:    Part A: {{value1}}<br>Part B: {{value2}}
Fragments: [raw("Part A: "), signal("value1"), raw("<br>")]
           ❌ "Part B: " and {{value2}} are lost

After:

Input:    Part A: {{value1}}<br>Part B: {{value2}}
Fragments: [raw("Part A: "), signal("value1"), raw("<br>Part B: "), signal("value2")]
           ✅ All content preserved

Test Plan

• Added regression test test_text_with_binding_after_void_element
• All 286 existing parser tests continue to pass
• cargo xtask check passes (license-headers, fmt, clippy, test)

Checklist

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I understand and have considered the performance impact of my changes.