Amazon Bedrock AgentCore Samples - [Content Improvement] - Improve runtime-context demo

This is feedback on the notebook on runtime-context:
01-tutorials/01-AgentCore-runtime/03-advanced-concepts/02-understanding-runtime-context/understanding_runtime_context.ipynb

**Describe the improvement request**
The current demo shows the use of session ID and reusing the same sessionID continues the conversation, but creating a new one starts a new conversation. This is great.

However, it introduces the concept of the 'user_id' parameter that confuses the matter. The explanation is not clear to state that the actual value of the 'user_id' has no effect on whether a session is reused or not. That only depends on the value of the sessionID.

Secondly, it gives the false impression that any user ID information must be passed as additional parameters. This can be a security concern if the agent developer mixes up the userId from the authentication token with the value of this parameter.

**What are your suggestions?**
The right way to show two users is to actually create two users in Cognito User Pool, obtain a token per user and use each to invoke the agent.

The Agent must use information from the authentication token to display user name and not the value of an arbitrary parameter. Please reinforce this secure practice in this tutorial.

Start with 01-tutorials/01-AgentCore-runtime/03-advanced-concepts/02-understanding-runtime-context/understanding_runtime_context.ipynb and run through the existing runtime-context demo. Clarify that sessionID controls conversation reuse, distinguish it from user_id, and show two Cognito users invoking the agent with separate tokens. Done means the notebook demonstrates displaying identity from the authentication token and reinforces the secure practice described in the issue.

Written by the indexing model from the issue text.