Fix #3100: Hoist whitespace outside markdown emphasis markers

[JiuqingSong]

Summary

• Fixes convertContentModelToMarkdown() may generate invalid markdown #3100. The markdown emitter wrapped segment text with **/*/~~ verbatim, so a text segment with surrounding whitespace produced invalid CommonMark (e.g. **world **how instead of **world** how).
• textProcessor in createMarkdownParagraph.ts now hoists leading/trailing whitespace outside the emphasis markers and skips wrapping for whitespace-only segments.
• One existing test in createMarkdownBlockgroupTest was asserting the buggy > *text *[link](...) output — updated to the correct > *text* [link](...).

Test plan

• yarn test:fast --testPathPattern=roosterjs-content-model-markdown — 199/199 pass
• yarn eslint — clean
• Five new unit tests cover: trailing whitespace inside bold, leading whitespace inside italic, surrounding whitespace inside strikethrough, combined bold+italic, and whitespace-only segments.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Fixes issue #3100 where the markdown emitter could produce invalid CommonMark by wrapping segments that contain leading or trailing whitespace inside emphasis markers (e.g. **world **how). The textProcessor in createMarkdownParagraph now hoists surrounding whitespace outside the **/*/~~ markers and returns whitespace-only segments untouched. An existing test that asserted the buggy output was updated, and five new unit tests cover the fix.

Changes:

• Refactor textProcessor to extract leading/trailing whitespace via regex and apply emphasis markers only around the non-whitespace core, including when a link is present.
• Update the previously incorrect > *text *[link](...) assertion to > *text* [link](...).
• Add unit tests for bold trailing whitespace, italic leading whitespace, strikethrough surrounding whitespace, combined bold+italic, and whitespace-only segments.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 2 comments.

File	Description
packages/roosterjs-content-model-markdown/lib/modelToMarkdown/creators/createMarkdownParagraph.ts	Reworked textProcessor to hoist whitespace outside emphasis markers and skip wrapping whitespace-only segments.
packages/roosterjs-content-model-markdown/test/modelToMarkdown/convertContentModelToMarkdownTest.ts	Added five regression tests covering the new whitespace-hoisting behavior.
packages/roosterjs-content-model-markdown/test/modelToMarkdown/creators/createMarkdownBlockgroupTest.ts	Updated an existing assertion that encoded the buggy markdown output.

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