allozaur/feat/chat form contenteditable (#26717)
* feat: Add contenteditable tokenizer for badge/code-chip chat input
* feat: Add source-space undo/redo history for the rich input
* feat: Split text glued to a closing code fence onto its own line
* feat: Add ChatFormContenteditable rich input renderer
* feat : wire the contenteditable into ChatForm with auto-switch gating
