fix(html): keep a pipe in a table cell from adding a column

[Lukas (L4XB)]

The bug

A Markdown table row is split on every unescaped pipe. _CustomMarkdownify inherits markdownify's convert_td/convert_th, which write the cell's text through unchanged, so a cell that holds a pipe silently adds a column.

On main (eb31b5c), an .xlsx with a part number in it:

| Product | Spec |
| --- | --- |
| Cable | USB-A|USB-C |
| Hub | 4 ports |

The header has 2 columns and the Cable row now has 3. Read back, the row is Cable / USB-A / USB-C — 4 ports is still in column 2 while USB-C has moved to a column that does not exist, so the table no longer lines up from that row on.

It is not an exotic cell value. A pipe shows up in part numbers, shell commands (grep -E 'a|b'), regex alternations, "either/or" notes, and anything pasted out of a terminal.

This reaches Markdown through the shared HTML converter, so every format that routes through it is affected:

format	path
.html	directly
.docx	mammoth → HTML → _CustomMarkdownify
.xlsx / .xls	sheet_to_html → _CustomMarkdownify
.epub, .ipynb HTML output, RSS	same converter

The fix

convert_td/convert_th escape the pipes in the cell's text before markdownify lays the row out:

_TABLE_CELL_PIPE = re.compile(r"(?<!\\)(\\*)\|")

def _escape_table_cell(text: str) -> str:
    return _TABLE_CELL_PIPE.sub(lambda match: match.group(1) * 2 + r"\|", text)

The regex matches the run of backslashes in front of the pipe rather than the pipe alone. That is the part that matters: a cell already containing a\|b would otherwise come out as a\\|b, where the cell's own backslash escapes the one we added and the pipe splits the row anyway. Doubling the run first makes a\|b into a\\\|b, which reads back as the literal a\|b.

A pipe outside a table is untouched — only the two cell converters call it.

Tests

packages/markitdown/tests/test_table_cell_pipe.py. The helper reads the produced table back the way a Markdown reader does (split on unescaped pipes, then resolve backslash escapes) and asserts on the cells, not on a fixed string, so it tests the property rather than the formatting.

9 cases: a part number, a shell command, a doubled ||, a cell with its own a\|b, a header cell, an .xlsx end to end, plus 3 guards — C:\path, a plain cell, and a pipe in a paragraph outside any table.

Measured:

	result
with the change	9 passed
source change stashed, tests kept	6 failed, 3 passed

Full suite: 851 passed, 14 skipped (unchanged). black clean.

Note: this touches the same file as #2519, which adds a URL helper near the top. They are independent changes; I will rebase whichever one lands second.