[Repo Assist] Split HtmlDocument operations out of HtmlOperations.fs

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🤖 This PR was created by Repo Assist, an automated AI assistant.

Part of #1677 — splitting large files in the codebase.

What's changed

HtmlOperations.fs was 1169 lines and contained two conceptually separate concerns:

• HtmlAttribute and HtmlNode operations (with their C# extension wrappers)
• HtmlDocument operations (with its C# extension wrappers)

This PR splits the HtmlDocument module, HtmlDocumentExtensions type, and HtmlExtensions module into a new file HtmlDocumentOperations.fs, placed immediately after HtmlOperations.fs in the project.

Result:

• HtmlOperations.fs: 1169 → 865 lines (HtmlAttribute + HtmlNode operations)
• HtmlDocumentOperations.fs: new, 316 lines (HtmlDocument operations)

This follows the existing pattern in the codebase where HtmlNode.fs holds the HtmlNode type definition and HtmlOperations.fs holds operations on it — now HtmlDocumentOperations.fs holds operations on HtmlDocument.

Note: This issue should remain open per the maintainer's request. Not using "Fixes" or "Closes" for issue #1677.

Test Status

• ✅ Build: succeeded (0 errors)
• ✅ All tests passed: 489 design-time, 283 integration, 26 C# interop, 2 reference (total 800+)
• ✅ Fantomas format check: passed

Generated by Repo Assist

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• Fixes Split large files in this codebase #1677

Generated by Repo Assist for issue #1677

To install this agentic workflow, run

gh aw add githubnext/agentics/workflows/repo-assist.md@afb00b92a9514fee9a14c583f059a03d05738f70