[FEATURE] Improve Ag-UI Agent Features

[joshuali925]

Is your feature request related to a problem?
I was testing #4409 using https://github.com/jiapingzeng/ml-commons/tree/3.4-jpz, found two issues

1. The MLAGUIAgentRunner hard codes to only include the latest tool result, but if agent needs to use multiple tools, it will lose previous tool results and won't have enough information to answer. This makes the agent not possible to use multiple tools for one question.
2. When sending messages to ag-ui agent execute API, seems system role messages are ignored and only instructions in messages with the user role appear to be seen by the model.

What solution would you like?

Include all tool results and support system messages

What alternatives have you considered?
A clear and concise description of any alternative solutions or features you've considered.

Do you have any additional context?
opensearch-project/OpenSearch-Dashboards#11201

[dhrubo-os]

@jiapingzeng could you please take a look?

[jiapingzeng]

please assign to me