Fix nested display:none span handling in CustomHtmlParser

[HakanIST]

Summary

Fix for T426910: Citation previews for the {{Cite}} template on Polish Wikipedia render garbled author names in the reference dialog.

Root Cause

CustomHtmlParser.CustomTagHandler used a single lastHiddenSpanPos variable to track <span style="display:none"> elements. When a hidden span contained child spans, the first inner </span> close event would trigger output deletion and reset the tracker. Subsequent sibling spans inside the hidden parent then rendered their content as visible text.

Polish Wikipedia's {{Cite}} template uses nested display:none spans to toggle between full author names and initials:

<span class="cite-name-after" style="display:none">
    <span>\&nbsp;</span>           <!-- closes first, resets tracker -->
    <span class="cite-name-full">Guy</span>    <!-- now visible (bug) -->
    <span class="cite-name-initials">G.</span> <!-- now visible (bug) -->
</span>

This produced output like Guy G. Mansell Guy G. instead of Guy Mansell.

Fix

Replace the single-variable approach with depth-tracked nesting:

• hiddenSpanDepth counts nesting depth inside a display:none span
• hiddenSpanStartPos records the output position where the outermost hidden span opened
• Nested span open/close events adjust the depth counter without triggering premature deletion
• When the outermost hidden span closes (depth returns to 0), all content appended since it opened is deleted

Testing

• Added CustomHtmlParserTest with 9 test cases covering simple, nested, deeply nested, sequential, and empty hidden spans, plus the exact {{Cite}} template pattern from Polish Wikipedia
• Verified existing StringUtilTest still passes
• All tests pass locally with Robolectric