fix(html): escape pipe characters in table cells

[Soroush Ahmadi (soroush5)]

Fixes #2438.

markdownify passes table cell text through untouched, so a literal pipe in a cell splits the Markdown row into phantom columns. This adds convert_td/convert_th overrides on _CustomMarkdownify that escape unescaped pipes, mirroring the base logic otherwise (colspan handling, newline collapsing).

The match is parity-correct: already-escaped pipes are left alone, so converters that pre-escape before reaching this layer (XLSX via _escape_sheet, PPTX in #1785) are unaffected — no double escaping. DOCX tables are fixed as well since they convert via the HTML converter (verified end-to-end with a .docx containing a pipe in a table cell).

Tests: 3 new cases in test_html_converter.py (cell pipe, header pipe, no double-escape). The first two fail before the fix, all pass after. Full package suite green: 649 passed, 4 skipped.

[Soroush Ahmadi (soroush5)]

@microsoft-github-policy-service agree

[Erik Rekola (erekola)]

I tested d5d07b1 with four two-column HTML tables and rendered the Markdown with markdown-it-py 4.2.0, with table support enabled. The first cell contained a plain pipe, a pipe inside inline code, a pipe in link text or an HTML pipe entity. The second cell was always end. All four lose that control cell on current main. On this PR all four keep both cells, and the inline code and link still render as code and a link. The converter dependency was markdownify 1.2.3 on Windows with Python 3.13.15.

The inline-code case is a useful extra regression fixture because checking the rendered cells catches content loss that counting separators misses:

<table><tr><th>Value</th><th>Control</th></tr><tr><td><code>a|b</code></td><td>end</td></tr></table>

Expected body cells after rendering are <td><code>a|b</code></td> and <td>end</td>. I ran four parameterized assertions against main and the PR checkout: four failed on main and four passed on the PR. I haven't tested DOCX or the full suite. AI assistance was used for the fixtures, checks and this comment.