[tldraw] Preserve rich text formatting when pasting tldraw content into editable shapes

## Problem When pasting copied tldraw shape content into an editable text shape, the rich text formatting (bold, italic, highlighting, etc.) is lost. The current implementation (PR #8192) extracts only the `text/plain` fallback from the clipboard, which strips all formatting. For example, if you copy a shape whose label contains **bold** or *italic* text and paste it into another editable shape, the formatting is discarded and only plain text is inserted. ## Context This was identified during review of #8192. The current fix correctly prevents pasting raw tldraw JSON data into text shapes, but as a tradeoff it downgrades rich text to plain text. ## Proposed solution When pasting tldraw clipboard data into an editable rich text shape: 1. Parse the tldraw clipboard data from the `text/html` payload (the `<div data-tldraw>` wrapper contains compressed JSON with shape data) 2. Extract the `richText` (TipTap JSONContent) from the source shape's props 3. Insert the rich text content i