fix: allow agent host to define a custom host and port

[ctorrao]

Description

I was trying to run locally two instances of the Demo UI + Agent Host, and noticed that even when I defined a different env var "A2A_UI_PORT" to run the Demo UI in another port everything was working but the state was shared between the two UI instances, because they pointed to the same Host Agent (hardcoded to run in localhost:12000).

To solve this I propose this change in the Demo UI code, to allow to also run two instances of Agent Host, each per instance of Demo UI.

This follows the same logic that was implemented here:

A2A/demo/ui/main.py

Lines 155 to 160 in 35f51ce

	# Setup the connection details, these should be set in the environment
	host = os.environ.get('A2A_UI_HOST', '0.0.0.0')
	port = int(os.environ.get('A2A_UI_PORT', '12000'))
	# Set the client to talk to the server
	host_agent_service.server_url = f'http://{host}:{port}'