[EuiComboBox] Improve changing selection by highlighting current text

[mdefazio]

Describe the solution you'd like When changing options in the combobox with only single selection available, the current option should be highlighted so I can start typing the new option.

https://github.com/user-attachments/assets/c9cb625d-739d-449e-808e-b35f3c5f6e31

Happy to discuss some alternatives or pushback on this. I acknowledge that using the keyboard to select all is easy here. But it does trip folks up for a second when interacting with this component.

Desired timeline This occurs on the add field mappings flows, and these have a lot of focus within the company at the moment.