Escape pipes in HTML/DOCX/XLSX table cells

[Aurora (Zhao0335)]

Problem

A literal | in an HTML table cell is data, not a column separator, but markdownify emitted it raw, so converted rows gained phantom columns.

#2438 — HTML/DOCX tables. DOCX is affected because it converts via the HTML layer.

#2436 — XLSX/XLS tables. Both Excel converters build an HTML table with DataFrame.to_html and run it through HtmlConverter, so they share the same bug.

Repro (HTML, current main):

import io
from markitdown import MarkItDown

html = (
    '<table><thead><tr><th>Name</th><th>Note</th></tr></thead>'
    '<tbody><tr><td>Alice</td><td>Has a | pipe</td></tr></tbody></table>'
)
print(MarkItDown().convert_stream(
    io.BytesIO(html.encode()), file_extension='.html').markdown)
# Before: | Alice | Has a | pipe |   (broken)
# After:  | Alice | Has a \| pipe |  (correct)

Repro (XLSX):

import io
import pandas as pd
from markitdown import MarkItDown

buf = io.BytesIO()
with pd.ExcelWriter(buf, engine="openpyxl") as w:
    pd.DataFrame({"a": ["x|y"], "b": ["2"]}).to_excel(w, index=False)
buf.seek(0)
print(MarkItDown().convert_stream(buf, file_extension=".xlsx").markdown)
# Before: | x|y | 2 |
# After:  | x\|y | 2 |

Fix

Override convert_td / convert_th in _CustomMarkdownify to apply the same cell escaping the CSV converter already uses (#2266 / #2464):

• Double any run of backslashes immediately before a pipe, then escape the pipe ((?<!\\)(\\*)\|).
• Collapse embedded newlines to spaces so the row stays on one line.

Because XLSX/XLS and DOCX both convert through this HTML layer, one change covers both Excel formats and DOCX without touching those converters.

Tests

• test_html_converter.py: pipes in cells and headers, backslash+pipe, plain cells unchanged, newline collapse, colspan still expands.
• test_module_misc.py: XLSX pipe in cell and header.

pytest packages/markitdown/tests/test_html_converter.py packages/markitdown/tests/test_module_misc.py -k "html or pipe or xlsx or csv or table" — 44 passed.

Fixes #2438
Fixes #2436

[Aurora (Zhao0335)]

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