We should align Combobox with the last updates in Lookup from T362650, as both components involve typing text within an input and selecting an item from the menu.
This seems to already be the current functionality. You can test it at https://doc.wikimedia.org/codex/latest/components/demos/combobox.html#basic-usage. I think this is because Comboboxes actually take their value from the input. Maybe @AnneT can clarify.
Reopening this task and updating the task description. We should update the Form field demo in Combobox to include warning and error inline messages when the entry is non-valid as we are doing in T362650: Lookup: address usability issues.
With Lookup, the important piece of data that you're collecting is the selection. With Combobox, it's the input - a user can either type in something, or use the input to search for an item in the menu then select it. So, Combobox can accept arbitrary input or a selection. Therefore, we should not implement the same selection behavior or error/warning messages as in Lookup, since arbitrary input is valid.