fix(table): preserve hardBreak nodes during markdown cell serialization

[sahiee-dev]

Changes Overview

Fixes data loss where <br> tags inside markdown table cells are silently dropped during re serialization via editor.storage.markdown.getMarkdown(), collapsing multi line cell content into a single line.

Implementation Approach

extension-hard-break correctly serializes <br> as \n (CommonMark spec). The data loss occurs in extension-table's collapseWhitespace(), which strips all \s+ including that \n before writing the cell back to markdown.

The fix intercepts hardBreak's exact output ( \n) and converts it to <br> before collapseWhitespace() runs:

// COUPLING WARNING: Tiptap's extension-hard-break hardcodes its markdown output to `  \n`
// (defined in packages/extension-hard-break/src/hard-break.ts).
// Because node renderers don't receive contextual metadata (like "inside a table cell"),
// the hardBreak node has no way to change its output to `<br>` contextually.
// We intercept `  \n` here before collapseWhitespace blindly strips it.
// If extension-hard-break ever changes its serialization string, this MUST be updated.
const HARDBREAK_MARKDOWN_PATTERN = '  \n'
raw = raw.replaceAll(HARDBREAK_MARKDOWN_PATTERN, cellSep)

I investigated passing table cell context down to hardBreak so it could emit <br> directly, but h.renderChildren does not propagate meta across recursion boundaries that would require a broader MarkdownManager refactor.

Also updated DEFAULT_CELL_LINE_SEPARATOR from \u001F to <br> for consistency with the preserved output format.

Testing Done

Added a round trip test in packages/extension-table/__tests__/tableMarkdown.spec.ts, with HardBreak registered in the test markdown manager:

it('should preserve <br> line breaks inside table cells during round-trip serialization', () => {
  const markdown = `| desc |\n| --- |\n| foo<br>bar |`
  const parsed = markdownManager.parse(markdown)
  const serialized = markdownManager.serialize(parsed)
  expect(serialized).toContain('foo<br>bar')
  expect(serialized).not.toContain('foo bar')
})

All extension-table tests pass.

Verification Steps

1. Open the Tiptap markdown editor
2. Paste the following markdown:

|    desc     |
|   :-----:   |
| foo<br>bar  |

3. Call editor.storage.markdown.getMarkdown()
4. Confirm the output contains foo<br>bar and not foo bar

Additional Notes

packages/markdown/__tests__/paragraph.spec.ts has a pre existing failure on the upstream develop branch unrelated to this change verified by running it against a clean develop branch before and after applying this fix.

Checklist

• I have created a [changeset](https://github.com/changesets/changesets) for this PR if necessary.
• My changes do not break the library.
• I have added tests where applicable.
• I have followed the project guidelines.
• I have fixed any lint issues.

Related Issues

Closes #7731

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[bdbch]

Closing this in favor for #8012