Implement JsonEventOutputHandler in C++ framework

[kovtcharov]

Context

Part of the C++ WebUI Integration milestone. See docs/plans/cpp-webui-integration.md for full architecture.

Description

Create a new OutputHandler subclass in the C++ framework that emits JSON-line events to stdout. Each line is a JSON object matching the SSE event format the WebUI frontend already consumes.

Files:

• cpp/include/gaia/json_event_handler.h
• cpp/src/json_event_handler.cpp

Event Mapping

Every OutputHandler virtual method maps to one JSON event:

Method	Event JSON
printStepHeader(n, total)	{"type": "step", "step": n, "total": total, "status": "started"}
printStreamToken(token)	{"type": "chunk", "content": "token"}
printStreamEnd()	(internal state, no output)
printThought(text)	{"type": "thinking", "content": "text"}
printGoal(text)	{"type": "status", "status": "working", "message": "text"}
printPlan(plan, current)	{"type": "plan", "steps": [...], "current_step": n}
printToolUsage(name)	{"type": "tool_start", "tool": "name"}
prettyPrintJson(args, "Tool Args")	{"type": "tool_args", "tool": "name", "args": {...}, "detail": "..."}
printToolComplete()	{"type": "tool_end", "success": true}
prettyPrintJson(result, "Tool Result")	{"type": "tool_result", "title": "...", "summary": "...", "success": true}
printFinalAnswer(text)	{"type": "answer", "content": "text", "steps": n, "tools_used": n}
printError(msg)	{"type": "agent_error", "content": "msg"}
printWarning(msg)	{"type": "status", "status": "warning", "message": "msg"}
printInfo(msg)	{"type": "status", "status": "info", "message": "msg"}
printCompletion(steps, limit)	{"type": "status", "status": "complete", "steps": n}

IMPORTANT: Do NOT Emit done Event

The done SSE event ({"type": "done", "message_id": N}) is emitted by the Python wrapper after the C++ subprocess exits and the message is saved to the database. The C++ JsonEventOutputHandler must NOT emit done — it doesn't have access to the database message ID.

Tool Result command_output Extraction

For tool results containing shell command output, parse the result JSON to extract structured command_output:

{
  "type": "tool_result",
  "title": "Tool Result",
  "summary": "...",
  "success": true,
  "command_output": {
    "command": "netsh wlan show interfaces",
    "stdout": "...",
    "return_code": 0
  }
}

C++ agent tools typically return JSON with "tool", "command", "output", and "exitCode" fields. The handler should map these to the command_output structure the frontend expects.

Distinguishing Tool Args vs Results

The prettyPrintJson(data, title) method is called with:

• title = "Tool Args" → emit as tool_args event
• title = "Tool Result" → emit as tool_result event
• Other titles → emit as generic tool_result

This is already how the C++ agent uses it (verified in agent.cpp lines 501, 508).

Internal State Tracking

The handler must track:

• currentTool_ — name of the tool currently executing (set by printToolUsage)
• stepsTaken_ — step counter (incremented by printStepHeader)
• toolsUsed_ — tool counter (incremented by printToolComplete)
• These are needed for the answer event's steps and tools_used fields

Acceptance Criteria

• JsonEventOutputHandler implements all 19 OutputHandler virtual methods
• Each method emits exactly one JSON line to stdout (flushed immediately)
• JSON format matches the SSE event types the WebUI expects
• Tool args and results correctly distinguished via title parameter
• command_output structured format for shell tool results
• Internal step/tool counters maintained for answer event stats
• Does NOT emit done event (Python wrapper responsibility)
• Unit tests validate JSON output for each method
• Registered in CMakeLists.txt build

Dependencies

• PR feat(cpp): SSE streaming response support for C++ agent framework #518 must be merged first (provides printStreamToken() / printStreamEnd())