fix: filling values for ComboBoxes and MultiSelectComboboxes.

[czp13]

Description

Based on slack discussions (Leif's suggestions): values can only be AI-filled into ComboBoxes and MultiSelectComboBoxes, if:

• The values are already present in the items list OR
• The values aren't in the items list, but the 'allow custom values' option is set to true.

Fixes # (issue)

• https://github.com/vaadin/form-filler-addon/issues/125

Type of change

• [x] Bugfix
• [ ] Feature

Checklist

• [x] I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• [x] I have added a description following the guideline.
• [x] The issue is created in the corresponding repository and I have referenced it.
• [x] I have added tests to ensure my change is effective and works as intended.
• [x] New and existing tests are passing locally with my change.
• [x] I have performed self-review and corrected misspellings.

Additional for Feature type of change

• [x] Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.

[czp13]

It was tested with the following cases:

1. allowedCustomValue false: 1.A English (values also existing in item List) the values were filled 1.B Hungarian not filled the ComboBox or MultiSelectCombobox (picture below):

2. allowedCustomValue true: 2.A and 2.B in both cases the values were always filled in the ComboBox and MultiSelectCombobox (picture below):

[czp13]

Added covering tests and fixed other review comments too. Thank you for your suggestions once more @mshabarov! 🙇

Unfortunately, parameterized tests are not supported in Junit4 (in Junit5 it will be), and I did not want to add another dependency just for this, so the tests are ugly because of it.

With Parameterized tests, it would have been a fraction of the code and cleaner as well :( Anyway, I added most use cases I could think of for Comboboxes and MultiSelectComboboxes.