Fix bug where user can't type disabled option into combobox

[m-akinc]

Pull Request

🤨 Rationale

Fixes #2252

👩‍💻 Implementation

The options getter should not revert to returning all the options (i.e. _options) when filteredOptions is empty. Doing so can lead to setting a selectedIndex that points to an option that is filtered out or disabled.

The expected behavior now is that a disabled option functions exactly like an enabled option, except that it cannot be clicked, cannot be focused in the dropdown, and appears dimmed. Like enabled options, t can be filtered to (if the input text matches it), and the user can set the control's value to it by typing it in fully.

🧪 Testing

Added new tests

✅ Checklist

• I have updated the project documentation to reflect my changes or determined no changes are needed.

[rajsite]

@atmgrifter00 it looks like there were changes since your last review, could you give it another check JIC?

[jattasNI]

Resetting my vote because I'd like to do one more round of testing based on the final implementation after the last bit of feedback is addressed