fix(cli): propagate url_prefix to FastAPI root_path

[luissilva-mollie]

Fixes #7070

Problem

--url_prefix is documented as making generated URLs work correctly when adk web is mounted behind a reverse proxy under a path prefix, but it's currently only used to set the dev-ui's backendUrl and the /dev-ui/ redirect target. It's never passed to the FastAPI app as root_path, so /docs and /openapi.json (and any other Starlette-generated URL) are always generated relative to /, breaking them when the app is proxied under a prefix that gets stripped before reaching the app.

Fix

Derive root_path from url_prefix via urllib.parse.urlparse(url_prefix).path and pass it into FastAPI(...). This handles both forms url_prefix can take today: a bare path (e.g. /adk, per the CLI help text) and a full absolute URL (e.g. https://host/adk, which some deployments pass since the same value is reused for the frontend's absolute backendUrl).

Testing

Verified locally with a minimal FastAPI app that setting root_path causes /docs to reference /<prefix>/openapi.json instead of /openapi.json:

from fastapi import FastAPI
from fastapi.testclient import TestClient
app = FastAPI(root_path="/agents/mcpolliegpt")
c = TestClient(app, base_url="http://testserver")
r = c.get("/docs")
# embedded openapi.json url is now "/agents/mcpolliegpt/openapi.json"

No existing tests cover url_prefix/root_path behavior in tests/unittests/cli/test_fast_api.py; happy to add coverage if maintainers can point me at the preferred test harness pattern for ApiServer.get_fast_api_app/run_server.