Render follow-up directive labels in the TUI and copied responses - #49089

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## Why

Follow-up directives contain display labels and prompt attributes. Show the labels without exposing directive syntax or prompt text in rendered responses and streaming previews.

## What changed

- Render `:codex-followup[label]{prompt="..."}` as its label, preserving inline Markdown styling and source offsets.
- Replace recognized follow-up directives with their labels when copying whole responses, while retaining the original source.
- Withhold unfinished directives in rich streaming previews and avoid cropping into prompt attributes when the preview window advances.
- Preserve literal code, escaped directives, and malformed source when rendering completed messages or copying responses.

## Testing

Add regression coverage for label formatting, nested brackets and backticks, literal and malformed directives, whole-response copying, and streaming holdback across long prompts and preview window boundaries.

GitOrigin-RevId: dce61d1164de0a576fe95fb2c67b177a1ed414e8