fix: preserve indentation around XML comment code blocks

[vicancy]

Summary

Follow-up to #10966. Keep the indentation when inserting blank lines between adjacent block elements and around inline child elements. Reuse the existing indentation helper, extending its search to the enclosing element when the child has no preceding newline of its own.

The separators previously inserted as bare \n\n created a zero-indented line. Common-indent removal then left the surrounding XML documentation indentation in place, so Markdown paragraphs became indented code blocks. Compact nested examples and lists had the same problem.

Reproduction

<summary>
    <para>Before</para><code>var x = 1;</code>
    After **bold**.
</summary>

After #10966, the last line renders as <pre><code>After **bold**...</code></pre> instead of <p>After <strong>bold</strong>.</p>. The follow-up preserves the intended paragraph and the actual code block.

Validation

• Five regression cases failed before the fix and pass afterward. They exercise both sibling directions, compact nested examples (including leading text), and nested lists, with no indentation, four/eight spaces, and tabs.
• Tests call the real XML comment parser and Markdig HTML renderer in memory and assert paragraphs, emphasis, and the code block, rather than only comparing intermediate Markdown whitespace.
• All 186 Docfx.Dotnet.Tests tests pass on .NET 10, including the original fix: XML comment parse logics for block element that is adjacent to markdown content #10966 cases, with zero skips.
• Changed-file formatting and git diff --check pass.
• Based on merged main 8cddf17d90e4cfd8b9f2288662cd3142810eebc4; only these two files differ. No dependency or workflow changes.