feat: add Mastra support (adapter, message format, example app)

[askadityapandey]

What

Adds native Mastra support to OpenUI so developers can connect a Mastra agent backend to OpenUI's streaming renderer and chat interfaces without writing custom integration code.

Closes #295

Changes

• Added mastraAdapter to @openuidev/react-headless implements StreamProtocolAdapter, parses Mastra's AI SDK SSE stream into AGUIEvents including text deltas and tool call start/args/end events. Uses a persistent buffer across TCP chunk reads to handle split SSE frames correctly.
• Added mastraMessageFormat to @openuidev/react-headless implements MessageFormat, converts OpenUI's internal message history to Mastra's expected shape (handles text, image_url, and multipart content) and maps responses back.
• Exported both from the package's main index.
• Added examples/mastra-chat a standalone Next.js app following the same structure as examples/openui-chat, demonstrating a real Mastra Agent with createTool tools wired to OpenUI's <FullScreen /> via streamProtocol={mastraAdapter()} and mastraMessageFormat.

Test Plan

Verified locally examples/mastra-chat runs with OPENAI_API_KEY set, text streams correctly, tool calls are intercepted and rendered

pnpm --filter mastra-chat dev

Checklist

• I linked a related issue, if applicable
• I updated docs/README when needed
• I considered backwards compatibility

This is a purely additive change no existing adapters or formats are touched. Completely backwards compatible.

[rabisg]

I tried running it locally but couldn't. Please attach a screenshot/gif of a working request response in the PR description to help us with the review

[rabisg]

there shouldn't be any changes needed in this PR for any of the packages/**
can you please clean them up

[rabisg]

@askadityapandey We don't mind if you use AI coding tools - we all do. But reviewing each PR takes a lot of time and effort so please review your PR manually once before sending it for review.

There are a bunch of unrelated changes, deleting entire files in the core package - the motivation for which is unclear. I have a strong feeling you've not even looked at the generated code once before sending it for review

[rabisg]

Hi @askadityapandey - would love to get this merged. are you still working on this?

[askadityapandey]

Hi @askadityapandey - would love to get this merged. are you still working on this?

hey just updated the branch with the latest main and resolved the merge conflicts. should be ready to merge now, if there are no issues, feel free to merge!