feat(prompt-input): Add PromptInputRichtext with Lexical-powered markdown editing

[omaroubari]

Summary

• add PromptInputRichtext as a composer-style alternative to PromptInputTextarea
• keep rich text content synced with PromptInputProvider, submit it as markdown, and support links, markdown shortcuts, paste/attachments, and Enter-to-submit behavior
• add docs, example usage, and test coverage for the new editor flow
• solves PromptInput richtext support #296
• opens up the way for common LLM chat features like mentions and slash commands (re: Lexical typeahead plugin) Allow PromptInput to contain rich text input with clickable @mentions and /slash commands #179

Screenshot

Why Lexical

I chose Lexical because it gives us a lightweight, React-friendly editor with a solid plugin model and the exact primitives we needed for markdown shortcuts, autolinking, history, and controlled state sync without overhauling the existing PromptInput API.

Alternatives considered

I considered extending the existing PromptInputTextarea with more custom markdown behavior, and using the ProseMirror editor stack. Lexical felt like the best fit because it hits a good balance of flexibility, integration simplicity, and runtime weight for this composer use case. I haven't previously worked with ProseMirror, but I'll be happy to refactor if someone more experienced deems it a better fit. I think at some point this library will have a DocumentEditor component for editing artifacts like the ChatGPT canvas, which will require thinking well about the editor we will adopt.

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[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	@​lexical/​link@​0.42.0
	lexical@​0.42.0
	@​lexical/​markdown@​0.42.0
	@​lexical/​react@​0.42.0

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