Fix markdown horizontal rule detection

[Julia Roldi (juliaroldi)]

Summary

Update isContentMarkdown to recognize only exactly three dashes as a Markdown horizontal rule. This prevents four or more dashes from being classified as Markdown content and adds a regression test for the behavior.

How to test

1. Verify isContentMarkdown('---') returns true, while isContentMarkdown('----') returns false.

[github-actions]

PR Preview Action v1.8.1
Preview removed because the pull request was closed.
2026-08-14 18:29 UTC

[Copilot]

Pull request overview

This pull request tightens Markdown detection in roosterjs-content-model-markdown by making horizontal rule recognition stricter to avoid misclassifying plain text (4+ dashes) as Markdown during paste/content checks.

Changes:

• Update the horizontal-rule block pattern in isContentMarkdown to match exactly ---.
• Add a regression test ensuring ---- is not treated as a Markdown horizontal rule.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/roosterjs-content-model-markdown/lib/publicApi/isContentMarkdown.ts	Narrows the horizontal rule regex from “3+ dashes” to exactly three dashes.
packages/roosterjs-content-model-markdown/test/publicApi/isContentMarkdownTest.ts	Adds a regression test to ensure 4 dashes does not trigger Markdown detection.

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