[Repo Assist] Fix HtmlNode.ToString: preserve whitespace in elements nested inside <pre>

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Closes #1509

Problem

When HtmlNode.ToString() serializes a <pre> block, newlines and indentation can be incorrectly inserted into elements nested multiple levels deep inside <pre>. This corrupts output from syntax highlighters (e.g. shiki) that emit HTML like:

<pre><code><span class="line"><span style="color:red">let</span> <span style="color:blue">x</span></span></code></pre>

In this case, the <code> element (which has onlyText = false) would have a newline inserted after <code>, and the <span> elements inside <span class="line"> would also have newlines inserted between them — corrupting the output.

Root Cause

The serialize function in HtmlNode.fs handled the <pre> tag itself correctly (skipping formatting for its direct children via the isPreTag flag), but did not propagate any "inside pre" context to deeper descendants. When a non-pre element with multiple element children (onlyText = false) appeared inside <pre>, the canAddNewLine flag on sibling elements caused newLine calls.

Fix

Add an insidePre parameter to serialize. When true, all newline/indentation formatting is suppressed regardless of element type. The value is set to true when entering a <pre> element and propagated to all recursive calls:

-let rec serialize (sb: StringBuilder) indentation canAddNewLine html =
+let rec serialize (sb: StringBuilder) indentation canAddNewLine insidePre html =
     ...
     let isPreTag = name = "pre"
+    let nowInsidePre = insidePre || isPreTag

-    if canAddNewLine && not (onlyText || isPreTag) then
+    if canAddNewLine && not insidePre && not (onlyText || isPreTag) then
         newLine 0
     ...
-        if not (onlyText || isPreTag) then
+        if not insidePre && not (onlyText || isPreTag) then
             newLine 2
         for element in elements do
-            serialize sb (indentation + 2) canAddNewLine element
+            serialize sb (indentation + 2) canAddNewLine nowInsidePre element

The single-level case (spans directly inside <pre>) was already handled correctly by the existing isPreTag/onlyText checks; this fix extends that handling to all depths.

Test

Added regression test Maintain whitespace in deeply nested elements inside pre through round-trip in HtmlParser.fs covering the shiki-style deeply nested structure.

Test Status

• Build passes (dotnet build src/FSharp.Data.Html.Core/)
• All 2227 tests in FSharp.Data.Core.Tests pass (including the new regression test and all 10 existing whitespace-related tests)
• FSharp.Data.Tests could not be built: the WorldBank type provider requires network access to api.worldbank.org, which is blocked by the build environment's network proxy — unrelated to this change.

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