feat: Add AG-UI and A2UI protocol support for real-time streaming and rich UIs

[protolabs42]

Summary

This PR adds support for two complementary protocols that enable real-time communication and rich interactive UIs:

AG-UI Protocol (SSE Streaming)

• Real-time Server-Sent Events streaming at /agui endpoint
• Events for text chunks, tool calls, reasoning/thinking, and step lifecycle
• Compatible with CopilotKit and other AG-UI clients
• Spec: https://docs.ag-ui.com

A2UI Protocol (Rich Interactive UIs)

• Agents can generate declarative UI components (forms, cards, buttons, etc.)
• Components rendered by compatible clients (Flutter, Web, React Native)
• Full A2UI v0.8 standard catalog support (18/18 components)
• Spec: https://a2ui.org/specification/v0.8-a2ui/

Architecture

Agent Zero
    │
    ▼
┌─────────────────────────┐
│  A2UI Tool              │  Agent generates UI via a2ui:show, a2ui:form, etc.
│  (python/tools/a2ui.py) │
└───────────┬─────────────┘
            │ wrapped as CustomEvent
            ▼
┌─────────────────────────┐
│  AG-UI Endpoint         │  SSE streaming: text, tools, thinking, A2UI events
│  (/agui)                │
└───────────┬─────────────┘
            │ SSE
            ▼
        Client (Flutter, Web, etc.)

Files Added

AG-UI Layer:

• python/api/agui_endpoint.py - SSE endpoint
• python/helpers/agui_server.py - Queue management
• python/helpers/agui_validation.py - Request validation
• python/extensions/*/_15_agui_*.py - 8 streaming hooks

A2UI Layer:

• python/helpers/a2ui/ - Protocol package (types, builder, emitter, surfaces)
• python/tools/a2ui.py - A2UI tool for agents
• python/extensions/a2ui_action/_10_handle_action.py - User action handler
• prompts/agent.system.tool.a2ui.md - Tool documentation

Integration:

• run_ui.py - Mount /agui endpoint
• requirements.txt - Add ag-ui-protocol dependency
• docs/agui.md, docs/a2ui.md - Documentation

Verification

• AG-UI: 17/17 event types implemented (100%)
• A2UI v0.8: 18/18 standard components implemented (100%)
• All Python files pass syntax verification

See docs/VERIFICATION_REPORT.md for full compliance details.

Testing

1. Start Agent Zero with this branch
2. Test SSE streaming: curl -N -X POST http://localhost:port/agui -H "Content-Type: application/json" -d '{"messages":[{"role":"user","content":"Hello"}]}'
3. Test A2UI: Ask the agent to show a form or display information