Add AGUIMock — AG-UI protocol mock

[jpr5]

Summary

• New AGUIMock class implementing the AG-UI (Agent-to-UI) protocol — the SSE event streaming protocol CopilotKit uses between agents and the frontend
• All 33 AG-UI event types supported (lifecycle, text, tool calls, state, activity, reasoning, special, deprecated)
• 11 event builder functions for common patterns (text response, tool call, state update, reasoning, activity, etc.)
• Fluent registration API: onMessage(), onRun(), onToolCall(), onStateKey(), onReasoning(), onPredicate()
• Record & replay with tee streaming — proxy to real AG-UI agents, record event streams as fixtures, replay on subsequent requests
• Proxy-only mode (--agui-proxy-only) — forward without saving or caching
• Standalone and mountable modes (mount at /agui on LLMock)
• Config file, suite, and CLI support (--agui-record, --agui-upstream, --agui-proxy-only)

Use case

Frontend developers can test CopilotKit UI components without a running agent backend. Point the frontend at aimock's /agui endpoint and get deterministic AG-UI event streams from fixtures. For demos, use proxy-only mode with canned fixtures for repeatable scenarios while proxying unknown traffic to the real agent.

const agui = new AGUIMock();
agui.onMessage("hello", "Hi! How can I help?");
agui.onToolCall(/search/, "web_search", '{"q":"test"}', { result: "[]" });
agui.enableRecording({ upstream: "http://localhost:8000/agent", proxyOnly: true });

const llm = new LLMock();
llm.mount("/agui", agui);
await llm.start();

New files

• src/agui-types.ts — 33 event type definitions, fixture/match types
• src/agui-handler.ts — matching, 11 builders, SSE writer
• src/agui-mock.ts — AGUIMock class (Mountable)
• src/agui-recorder.ts — record/replay with tee streaming, proxy-only mode
• src/agui-stub.ts — barrel exports
• src/__tests__/agui-mock.test.ts — 32 tests

Modified files

• src/config-loader.ts — AGUIConfig support
• src/suite.ts — agui option in MockSuite
• src/index.ts — AG-UI exports
• src/cli.ts — AG-UI recording flags

Test plan

• 32 new tests (core 14, builders 5, edge cases 7, record/replay 6)
• All 2246 tests pass
• Build passes
• 3-round CR loop converged at 0 findings

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commit: 3f36d90