[EuiMarkdownFormat] Fix intraword underscore emphasis rendering

[acstll]

Summary

Adds a remark AST transformer plugin (remark_intraword_underscore) that reverses incorrectly applied emphasis/strong nodes when underscore delimiters are flanked by alphanumeric characters on both sides (intraword context).

remark-parse v8 does not implement the CommonMark specification rule that underscore delimiters which are both left-flanking and right-flanking (i.e. surrounded by word characters) cannot open or close emphasis. This plugin walks the parsed AST post-parse, detects these cases by checking sibling text nodes and verifying the delimiter is _ (not *) via source position, and flattens the nodes back to plain text with underscores restored.

This is a temporary workaround until remark-parse is upgraded to a version that natively handles this rule.

Why are we making this change?

Closes #9404

Salesforce API identifiers use underscores and double underscores extensively (e.g. SBQQ__OrderProductBookings__c, Custom_Field__c). EuiMarkdownFormat renders these with bold/italic formatting instead of preserving them verbatim, making the text unreadable and unusable for copy-paste.

Impact to users

🟢 Intraword underscore sequences that were previously (incorrectly) rendered as bold or italic will now display as plain text, matching CommonMark spec behavior. Legitimate emphasis using __bold__ or _italic_ with whitespace boundaries continues to work as expected. Asterisk-based emphasis (**bold**, *italic*) is completely unaffected.

QA

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General checklist

• Browser QA
  • Checked in both light and dark modes
  • Checked in both MacOS and Windows high contrast modes
    • (emulate forced colors if you do not have access to a Windows machine.)
  • Checked in mobile
  • Checked in Chrome, Safari, Edge, and Firefox
  • Checked for accessibility including keyboard-only and screenreader modes
• Docs site QA
  • Added documentation
  • Props have proper autodocs
  • Checked Code Sandbox works for any docs examples
• Code quality checklist
  • Added or updated jest and cypress tests
  • Updated visual regression tests
• Release checklist
  • A changelog entry exists and is marked appropriately
  • If applicable, added the breaking change issue label
  • If the changes unblock an issue in a different repo, smoke tested carefully
• Designer checklist

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[elasticmachine]

💚 Build Succeeded

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• Commit: 44864e7
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cc @acstll

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 44864e7

cc @acstll

[tkajtoch]

I was initially thinking we'd handle that outside remark but having it as a plugin is also a valid solution :D So far the changes look good. Let me know when the PR is ready for final review!

[acstll]

I was initially thinking we'd handle that outside remark but having it as a plugin is also a valid solution :D So far the changes look good. Let me know when the PR is ready for final review!

nice! i'll go through it again and add a small unit test, and then i'll open 🙏