[Feature]: send only last user message

### Pre-flight Checklist

- [x] I have searched existing issues and this hasn't been requested yet.

### Problem or Motivation

Hi! Your docs say that `RunAgentInput` should contain full conversation:
- https://docs.ag-ui.com/sdk/python/core/types#runagentinput
- https://docs.ag-ui.com/sdk/js/core/types#runagentinput

I think it would be great to support a lightweight regime when the UI sends request to agent by a last user message. It is often not necessary to send the whole `RunAgentInput`.

For example, I am working on the project, where I don't let frontend to modify the state and use latter one only for streaming progress from backend. In this setting, everything is needed (conversation history and agent state) is already available for agent, so sending it again seems unnatural, because it requires either checking that the persisted objects are in sync with the frontend-delivered input or just ignoring it.

Maybe I am wrong or not attentive enough to spot this feature in the docs, or interpreted these docs too strictly.

### Proposed Solution

dunno

### Alternatives Considered

_No response_

### Additional Context

_No response_