Fix XHS topic insertion order

[walker1211]

Summary

• Keep the XHS content editor cursor at the end before inserting separator newlines and topic tags.
• Prevent topic tags from being inserted into the middle of existing content.
• Add a regression test for preserving content order and avoiding empty paragraphs between title lines.

Test plan

• go test ./... -count=1
• Push hook: secret scan, gofmt, go vet, go test, build