[COMMS-572] Pasting rich text into CKEditor crashes it

[op-chomper]

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What does this PR change?

Fixes a crash where pasting formatted rich text into CKEditor (e.g. an AppSignal error page with a syntax-highlighted code block) threw CKEditorError: can't access property "replace", e6.data is undefined, which the watchdog answered by restarting the editor — wiping everything the user had typed.

The crash came from the code-block upcast converter assuming <code>'s first child was a text node (child.data.replace(...)). When the pasted code is wrapped in highlight <span>s, that child is an element and .data is undefined. It now extracts text via a recursive textContentOf() helper that only reads .data off real text nodes.

It also adds a defensive paste boundary so this class of failure can never crash the editor again.

What approach did you choose and why?

Two layers: (1) the targeted converter fix, which resolves the reported repro; and (2) a new OPPasteGuard plugin that runs the clipboard→model conversion in a try/catch before the pipeline's default insertion. On any converter error it aborts the rich paste, inserts the plain-text fallback, and emits op:clipboard-paste-error — so the editor stays alive and existing content is preserved even for a future, unknown converter bug. The companion OpenProject frontend change turns that event (and watchdog errors) into a user-facing flash instead of failing silently.

Screenshots

No visual changes (error-handling/flash messaging only).

Tests

Added unit tests for textContentOf covering plain text, single-text <code>, the regression case (element-wrapped <code>, asserts no throw), deep nesting, and empty elements. Manual verification: paste the AppSignal incident content into a meeting agenda editor — content pastes, no console error, previously typed text retained; Ctrl+Shift+V still works.

Merge checklist

• Rebuild the bundle (npm run build) and bump @openproject/commonmark-ckeditor-build in the OpenProject frontend — the fix isn't live until the generated build/ + src/ckeditor.js are regenerated.
• Companion OpenProject PR (component event handling + js.editor.error_paste_failed locale key) merged together.