[Feature Proposal]: adk graph - Visual Agent Inspector, Bi-directional Code Generator, & Live Debugger

### Summary
As agentic workflows scale, inspecting and debugging multi-agent delegation topology (`SequentialAgent`, `ParallelAgent`, `LoopAgent`, tools, memory) via terminal logs or raw Python code becomes challenging.

We propose adding a built-in CLI tool `adk graph` (and Web UI interface) that provides:
1. **Reverse Inspection / AST Parser (Python → Graph):** Reads existing ADK Python agent files and renders their multi-agent topology visually.
2. **Visual Builder & Code Generator (Graph → Python):** Drag-and-drop agent visual editor (integrating concepts from [ADKGraph](https://github.com/yuvaraj-97/ADKGraph)).
3. **Live Execution & Debugging Visualizer:** Highlights active agents and tools on the visual graph in real time as events stream over SSE/WebSockets during runner execution.

---

### User Experience / Proposed CLI

```bash
# 1. Inspect & debug an existing agent script
adk graph path/to/agent.py

# 2. Launch the visual agent editor to construct a new agent system
adk graph --new
```

When run, `adk graph` will:
1. Spin up a lightweight local inspection server.
2. Open an interactive web UI in the browser showing the agent topology.
3. Allow real-time step execution with visual active-node highlighting.

---

### Key Features

- **Bi-directional Editing:** Changes made in code update the graph visual; canvas edits compile directly back to production-ready `adk-python` code.
- **Hierarchical Orchestrator Containers:** Support nested visual group containers for `SequentialAgent`, `ParallelAgent`, and `LoopAgent`.
- **Live Event Debugger:** Visual node glow/state animation during active agent execution, tool calls, and LLM reasoning steps.

---

### Proposed Architectural Changes

- **CLI extension:** Add `graph` subcommand in `google.adk.cli`.
- **Inspector engine (`google.adk.cli.graph`):** Dynamic agent object visitor & Python AST static analysis module.
- **Web UI bundle:** Embedded lightweight React visual canvas server.

---

### Additional Context
This proposal is based on the open-source [ADKGraph](https://github.com/yuvaraj-97/ADKGraph) visual IDE built specifically for Google ADK. We are willing to contribute and maintain this feature via a Pull Request.

Is this feature aligned with the ADK roadmap? We welcome maintainer feedback before submitting our PR implementation!