Strip <img> tags from pasted HTML in TipTap editor

[rtibbles]

Summary

Strips every <img> tag from HTML pasted into the TipTap editor. Pastes from Google Docs, web pages, and other rich-text sources previously left remote https://... image references in the saved markdown, which blocked channel publish per #5895.

The transformer also subsumes the existing Word/Office cleanup that previously only ran on the toolbar Paste button — native Ctrl-V paste now gets the same treatment.

Users who want an image in the editor continue to use the existing upload flow (toolbar, drag-and-drop, or paste-image-data), which routes through imageProcessor.processFile and produces a checksum-backed image.

References

• Fixes Text editor - Cannot publish a channel because of an exercise with a lot of formatted text #5895 (P0: publish blocked by pasted images).
• Follow-up: Close Office and Google Docs paste sanitizer fidelity gaps #5896 tracks the broader Office/Google Docs paste sanitizer fidelity gaps that surfaced during this work (Word multi-level lists, Excel class-driven styles, GDocs paragraph fragmentation, etc.).

Reviewer guidance

Reproducer (matches #5895):

1. Open an exercise's TipTap editor.
2. Copy content from https://wiki.c2.com/?TextFormattingExamples (the page pcenov reproduced from).
3. Paste into the editor.
4. Image disappears; text and formatting survive.
5. Channel publishes without the Text editor - Cannot publish a channel because of an exercise with a lot of formatted text #5895 failure.

Verified locally:

• Paste from the c2 wiki page above against the dev server.
• Paste from Google Docs.

Also check:

• Toolbar Paste button (dropdown by formatting controls) — same behavior; previously had Word cleanup but no <img> strip.
• Word paste regression — MSO comments, mso-* styles, Mso* classes still dropped.
• Drag-and-drop and toolbar image insert unaffected (they go through imageHandler, not the paste path).

AI usage

Used Claude (Opus 4.7) to brainstorm scope, write a spec and implementation plan, and drive the TDD implementation (27 unit tests, then implementation). I made scope decisions throughout (strip-only vs fetch+upload; refactor into the transformer pipeline), reviewed each diff before commit, and ran the manual paste/publish verification against the dev server.

[marcellamaki]

Was able to follow the reviewer guidance here as described. Approving for @pcenov 's further testing/confirmation tomorrow! 😅