A2A agent cards do not support mounting adk as a fastapi sub-app and https.

[racinmat]

Describe the bug

This is related to #2072.
When adk-produced fastapi app is mounted as subapp, the URL is hardcoded here, and while it allows me to provide host and port, it would be better to let user pass whole url. And it's not possible to set https if the app is running under some ingress which uses TLS.

To Reproduce
Steps to reproduce the behavior:

Let's say I have the agent in agent_basic directory.

1. Make the agent

from pathlib import Path
import uvicorn
from fastapi import FastAPI
from google.adk.cli.fast_api import get_fast_api_app
app = FastAPI()
adk_app = get_fast_api_app(
    agents_dir=str((Path(__file__) / '..').absolute()),
    session_service_uri="sqlite:///:memory:",
    allow_origins=["*"],
    web=True,
)
app.mount("/adk", adk_app)
uvicorn.run(app, host="0.0.0.0", port=8081)

2. Run the code
3. Visit http://localhost:8081/adk/a2a/agent_basic/.well-known/agent.json
4. The resulting json shows "url":"http://localhost:8081/a2a/agent_basic" although the correct url is "url":"http://localhost:8081/adk/a2a/agent_basic"

Desktop (please complete the following information):

• OS: Windows 11
• Python version(python -V): Python 3.12.8
• ADK version(pip show google-adk): 1.7.0

Model Information:
Regardless of the model.

Additional context
I don't think anyting more is needed.