AG-UI Support

[jherr]

🎯 Changes

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm run test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

Summary by CodeRabbit

• New Features
  • Unified AG‑UI streaming protocol: RUN/TEXT/STEP/TOOL events, run‑scoped IDs, richer RUN_ERROR/RUN_FINISHED and TOOL_CALL lifecycle for clearer streaming and tool interactions.
• UI
  • New mock chat API and mock‑driven E2E flows; added Grok and OpenRouter options and Gemini 2.5 Pro; DEFAULT_MODEL_OPTION exposed.
• Documentation
  • Streaming guides and protocol docs rewritten to AG‑UI event terminology and examples.
• Tests
  • Tests and E2E suites updated/added to validate AG‑UI event sequences, ordering, payloads, and mock scenarios.

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