fix blueprint registering and double init

[T4rk1n]

Fix #3787

[camdecoster]

Rather than add these blueprint checks for each backend, what if you add a check in init_app to see if the same server has already been registered? If the same server is passed in, then it becomes a no-op. If it's a new server, then the init takes place with the new server.

If you go with the current per backend approach, certain calls would add duplicate handlers or attempt to register the same routes again:

self.backend.before_request(self._setup_server)
...
self._setup_routes()

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
2 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[camdecoster]

You could revert this change and get rid of this file from the PR.

[camdecoster]

I asked Claude to write a test that would look at the idempotency when calling init twice. This might be worth adding:

def test_init_app_idempotent_on_same_server():
    external = Flask("ext")
    app = Dash(__name__, server=False)
    app.init_app(external)
    app.init_app(external)  # must not raise
    # request still works
    client = external.test_client()
    assert client.get("/").status_code == 200