Bug: `get_fast_api_app()` fails with App class - "ValueError: Unsupported tool type"

**Describe the bug**
`get_fast_api_app()` crashes when agent modules contain the new `App` class (introduced in ADK 1.14.0+) during graph visualisation with "ValueError: Unsupported tool type".

**To Reproduce**

Steps to reproduce the behaviour:
1. Create agent module with both `root_agent` and `App` objects
2. Use `get_fast_api_app()` to serve the agent for Google Cloud Run deployment following [Deploy - Cloud Run - Python - gcloud - CLI](https://google.github.io/adk-docs/deploy/cloud-run#python---gcloud-cli)
3. Access the graph visualisation endpoint
4. See error: `ValueError: Unsupported tool type`

Minimal reproduction code:
```python
# agent/agent.py
from google.adk.agents.llm_agent import LlmAgent
from google.adk.apps import App
from google.adk.plugins.base_plugin import BasePlugin

class MyPlugin(BasePlugin):
pass

root_agent = LlmAgent(model="gemini-2.0-flash", name="agent", ...)
app = App(name="app", root_agent=root_agent, plugins=[MyPlugin()])
```

```python
# main.py
from google.adk.cli.fast_api import get_fast_api_app
app = get_fast_api_app(agents_dir="./agent", ...)
```

**Expected behaviour**
`get_fast_api_app()` should either handle App objects properly or ignore them during agent discovery. The web interface should work without crashing the graph visualisation endpoint.

**Screenshots**

**Desktop (please complete the following information):**
- OS: macOS
- Python version(python -V): 3.13
- ADK version(pip show google-adk): google-adk==1.14.1

**Model Information:**
- Are you using LiteLLM: No
- Which model is being used(e.g. gemini-2.5-pro): gemini-2.5-flash

**Additional context**
This is a regression introduced in ADK 1.14.0 with the new App class. The issue occurs in `agent_graph.py` line 77 where `get_node_name()` doesn't recognise App class objects, only `LlmAgent`/`Agent` types.

Workarounds tested:
- Renaming App variable ❌
- Standard project structure ❌
- Using only root_agent ❌ (plugins require App class)

This affects users upgrading to ADK 1.14.0+ who need App class features (plugins, multi-agent, etc.).

Potential solutions:
1. Update `get_node_name()` to handle App objects
2. Filter out App objects in agent discovery
3. Add full App class support to graph visualisation