fix(react-headless): robust SSE framing in agUIAdapter

[boffin-dmytro]

Summary

This PR makes @openuidev/react-headless’s default agUIAdapter() handle real-world SSE streams reliably.

The previous implementation parsed SSE by splitting each network chunk on \n, which can drop events when JSON/lines are split across chunks (common in production), and didn’t fully respect SSE framing rules.

What changed

• Hardened agUIAdapter() SSE parsing:
  • buffers across chunk boundaries (partial lines / partial JSON)
  • supports CRLF (\r\n)
  • supports blank-line delimited SSE events
  • supports multi-line data: fields (joined with \n per SSE spec)
  • ignores comment/keepalive lines
  • treats [DONE] as a terminator
  • best-effort by default: malformed events are skipped so streaming can continue
• Added regression tests covering:
  • chunk-splitting
  • CRLF + multi-line data:
  • malformed event skipping + continued parsing
• Added a small API reference note clarifying the expected SSE framing for agUIAdapter()

Why it matters

SSE chunk boundaries are arbitrary, and many servers emit CRLF and keepalives. With this change, AG-UI streaming is much more resilient under normal network conditions, preventing silent token/event loss and incomplete assistant messages.

Scope note

This PR intentionally focuses on agUIAdapter() (the default) to keep the change tight and low-risk. Similar SSE framing improvements could be applied to the OpenAI SSE adapters in a follow-up if maintainers want to consolidate parsing behavior.

Files changed

• packages/react-headless/src/stream/adapters/ag-ui.ts
• packages/react-headless/src/store/__tests__/createChatStore.test.ts
• docs/content/docs/api-reference/react-headless.mdx

Kinly review this update.