GO-6032 Protect angle brackets in HTML-to-blocks pipeline

[KirillSto]

Summary

• Add < and > to the protectedRunes escape list in anymark/escape.go
• When pasting HTML containing </> entities (e.g. from CodeMirror copy buttons, ViolentMonkey, IDE copy buttons), the html-to-markdown library decodes them to bare </>, which goldmark then misinterprets as HTML tags and silently drops
• The existing PUA sentinel escape mechanism already protects markdown-special characters ([, ], *, etc.) — this extends it to also protect angle brackets
• Fixes the issue for all paste targets (code blocks, paragraphs, etc.) since the fix is in the shared conversion pipeline

Test plan

• TestEscapeUnescapeAngleBrackets — round-trip escape/unescape preserves < and >
• TestHTMLToBlocks_AngleBrackets — HTML entities, arrow functions, JSX, generics, comparison operators all preserved through HTMLToBlocks
• TestClipboard_PasteHtmlWithAngleBrackets — end-to-end paste into code blocks and paragraphs preserves angle brackets
• All existing TestConvertHTMLToBlocks JSON test cases still pass (including underline <u> tags)
• All existing clipboard tests pass

🤖 Generated with Claude Code

[github-actions]

New Coverage	46.5% of statements
Patch Coverage	0.0% of changed statements (0/0)

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