fix: preserve Markdown hard line breaks

[Maciej Zmitrukiewicz (MaciejZet)]

Summary

• MarkItDown._convert was stripping every trailing space, so a CommonMark hard break (two or more spaces before a newline) became a soft break.
• Lines that end with at least two spaces are now kept. A single trailing space, a trailing tab, and a whitespace-only line are still removed. Runs of blank lines are still collapsed, so this does not overlap the fenced-code blank-line work in fix: keep blank lines inside fenced code blocks #2446 or the PPTX soft-break work in fix(pptx): convert soft line breaks instead of emitting a vertical tab #2513.
• HTML <br> and DOCX <w:br/> are emitted by markdownify as those two spaces. Existing expectations for that output now keep the hard break.

Test plan

• tests/test_markdown_whitespace.py: hard breaks, CRLF, backslash breaks, tabs, blank-line collapsing, fenced code, real .md input, and HTML <br>
• Updated HTML underline, DOCX break, and strikethrough expectations
• pytest tests in packages/markitdown on Python 3.12 with the all extra: 1024 passed, 14 skipped

[Maciej Zmitrukiewicz (MaciejZet)]

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