[RTE] Toolbar Copy should copy the selection's serialized content, not its DOM #6296

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Overview

The rich text editor's toolbar Copy button builds its HTML by cloning the selection's DOM, so every node view that holds data must special-case itself to copy correctly. Copy should produce the same HTML as ProseMirror's own clipboard serializer, the one Ctrl+C uses.

Complexity: Low
Target branch: unstable

Context

• handleCopy clones window.getSelection()'s range and patches it: <math-field> becomes a math-inline span, data-copy-ignore elements and Vue attributes are stripped, math-node-wrapper divs are unwrapped (useToolbarActions.js#L113-L186).
• The inline choice chip in Add inline choice chip and Insert action to the passage editor #6277 keeps a hidden copySource span holding its own renderHTML output only so this Copy picks it up (InlineChoiceChip/index.vue).
• Raised in review of #6277.

The Change

Toolbar Copy should write the HTML ProseMirror serializes for the selection (editor.view.serializeForClipboard(selection.content())), so a node view's DOM no longer decides what is copied.

Acceptance Criteria

• Toolbar Copy then paste of a selection containing a formula reproduces the formula.
• Toolbar Copy then paste of a selection containing an inline choice chip reproduces the chip with its options and correct answer.
• The math-field special case in handleCopy is gone.
• The inline choice chip's hidden copySource span is gone.
• Toolbar Copy and Ctrl+C put the same text/html on the clipboard for the same selection.
• Unit tests cover toolbar Copy of a selection with a formula and with a chip.

AI usage

A reviewer asked for this follow-up on #6277; Claude Code drafted it from that review thread and the current handleCopy code.