[BUG] Plotly Cloud devtool route crashes with FastAPI and Quart backends

The issue is in plotly_cloud/_devtool_hooks.py line 36 where flask.request.get_json() is called. First, understand how Dash registers routes for different backends (FastAPI, Quart, Flask). Look at the Dash server initialization and route registration logic. Check if the devtool route should be conditionally registered based on the backend. The fix likely involves checking the request context or using backend-agnostic request data access. Run the provided reproduction script to confirm the crash.

Describe your context

• OS: Linux on WSL2
• Python: 3.13.11
• Browser: N/A (server-side issue reproduced with backend test clients)

Dash 4.5.0rc0 / FastAPI environment:
dash          4.5.0rc0
plotly-cloud  0.5.0
fastapi       0.141.1

Dash 4.5.0rc0 / Quart environment (isolated):
dash          4.5.0rc0
plotly-cloud  0.5.0
quart         0.23.1

Describe the bug

With the FastAPI or Quart backend, the Plotly Cloud devtool registers POST /_plotly_cloud_publish, but its handler calls flask.request.get_json(). Neither backend provides a Flask request context, so this route returns HTTP 500:

File "plotly_cloud/_devtool_hooks.py", line 36, in plotly_cloud_publish_rpc
    data = flask.request.get_json()
RuntimeError: Working outside of request context.

This is exposed by default in Dash 4.5.0rc0 because #3947 made plotly-cloud a default dependency. To distinguish the dependency change from the underlying compatibility issue, I tested isolated environments:

Steps to reproduce

Install dash[fastapi,quart]==4.5.0rc0 and httpx, then run the following script in separate processes with fastapi and quart as the argument:

import asyncio
import sys

from dash import Dash, html
from fastapi.testclient import TestClient

backend = sys.argv[1]
app = Dash(__name__, backend=backend)
app.layout = html.Div("Hello")
app.enable_dev_tools(debug=True, dev_tools_ui=True)

if backend == "fastapi":
    response = TestClient(
        app.server, raise_server_exceptions=False
    ).post("/_plotly_cloud_publish", json={})
else:
    response = asyncio.run(
        app.server.test_client().post("/_plotly_cloud_publish", json={})
    )

print(response.status_code)  # 500 for both backends

Expected behavior

Enabling Dash Dev Tools with FastAPI or Quart should not register a Plotly Cloud route that crashes. The integration should support these backends or omit its devtool and route when they are unsupported.

Screenshots

Not applicable; this is a server-side error.