fix: don't leak HTML comment content into pasted paragraph text

[appflowy]

Summary

Cherry-pick of the fix already merged to main via #1220, onto fix/korean_ime since AppFlowy-Premium's pubspec.yaml pins appflowy_editor to this branch (it carries commits not yet on main).

• _parseDeltaElement's fallback branch called child.text on any non-Element DOM node, including dom.Comment (whose .text getter aliases its raw data). A real HTML comment embedded inline in pasted content — e.g. the internal tracking marker Google's Search AI Overview panel injects when you copy its answer text — would surface as visible garbage text in the editor instead of being silently dropped.
• Fix mirrors the pattern _parseElement already uses one level up: only extract text from dom.Text nodes; other node types are skipped.
• Cherry-picked cleanly from main (bb99582), no conflicts — only one other commit on this branch touches the same file and it edits an unrelated region.

Test plan

• flutter test test/customer/html_document_test.dart — new regression test passes
• Confirmed identical hang in test/render/image/image_upload_widget_test.dart pre-exists on fix/korean_ime even without this commit (unrelated pre-existing issue, not a regression from this change)

🤖 Generated with Claude Code

https://claude.ai/code/session_014Nps55LWFpd6VBJkPX5cap

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