Summary

Part of MST-12952: Flow users need to keep a {{ $vars.x }} reference as literal text in an inline-agent prompt, for example to pass one agent's prompt to another agent. Flow stores this as \{{ … }}, the same escape its expression fields already use. On load and save Flow converts between its string format and the editor's tokens, but plain-text copy/paste happens inside the prompt editor. There, any {{ … }} became a chip: the escape was ignored, and {{name}} pasted as an invalid chip.

This PR lets a host supply its own clipboard string format, and makes the default paste/copy handle the \{{ escape. The Flow side will follow in a flow-workbench PR once this is released.

Changes

• New optional PromptEditor props parseClipboardText / serializeClipboardTokens, passed to CopyPastePlugin. They replace the plain-text parse on paste and the plain text written on copy/cut. Lexical-JSON pastes between editors are unaffected. The plugin reads them through a ref, so inline callbacks don't re-register the commands.
• Default clipboardStringToTokens: the number of backslashes right before a {{ decides what it is. An odd number means an escaped literal {{; an even number means literal backslashes followed by a chip. The count is halved either way. A plain \{{ … }} is therefore literal text with the backslash dropped.
• Default tokensToClipboardString: a literal {{ in text is written as \{{. Backslashes right before a {{, or right before a chip, are doubled. This way any text survives a copy → paste, including text that ends in a backslash just before a chip.
• Two new stories: Clipboard escaping (default behaviour) and With host clipboard format (a host parser that only turns {{ vars.* }} into chips).

Flow

flowchart TD
    P[Paste plain text] --> H{parseClipboardText provided?}
    H -- yes --> HP[Host parser]
    H -- no --> D[clipboardStringToTokens]
    D --> E{"\{{ escaped?"}
    E -- yes --> T[Literal text]
    E -- no --> C[Chip]
    HP --> I[Insert tokens]
    T --> I
    C --> I

Testing

• pnpm lint passes (plus lint:css, lint:deps, check:dependencies)
• pnpm typecheck passes
• pnpm test passes including the new CopyPastePlugin.test.tsx (paste, plus copy/cut with a host serializer) and serialization round-trip cases
• pnpm test:visual: not applicable (no test:visual tasks defined)
• Manual testing / Storybook checked: both new stories render. This was a render smoke test only; I didn't click through them in the Storybook UI.
• Type-safe (no as any / type suppressions added)