[Feature Req] Talk to (AgentCore?) Agents

Whereas this sample currently enables talking directly to models, it'd be really useful if we could **also** configure it to connect to externally-hosted agents: For example hosted on AgentCore Runtime, but maybe other places too.

The issue names no files, tests, or entry points, so first map where the sample configures direct model conversations and how its Chainlit application handles requests. Define the supported external-agent configuration, including AgentCore Runtime, and verify that users can converse with a hosted agent without breaking direct model support.

Written by the indexing model from the issue text.