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