Add the prerequisite functionality in Dart `a2ui_core` to support the new `a2ui_agent` library

Prerequisite for: https://github.com/a2ui-project/a2ui/issues/2356

We need to make some changes to `a2ui_core` to make sure it provides all the necessary APIs that the new `a2ui_agent` library will need.

The issue is a prerequisite for a new library, but does not specify which APIs are needed. Start by examining the linked issue #2356 to understand the requirements for the a2ui_agent library. Then review the a2ui_core codebase to identify missing functionality. The work likely involves extending core interfaces or adding new methods to support agent interactions.

Written by the indexing model from the issue text.