Improvements of autocomplete

**As** user ,
**I want** the autocomplete feature to be optimized,,
**so that** so that it can be used with even more efficiency.

## Hints / Details

- Currently filtering for enums is not provided and the BE returns always all. The FE however display 20 entries. It has to be clarified how to handle a case with more then 20 enums.
- After filtering for any existing entry and select all of them by clicking on according checkbox, further filter entries for same column is possible but the "select all" checkbox is still selected. Any click on it will deselect previous filter.
- Currently the EdgeCase of searching for already selected items results in "Items not found" Error message.
- Filtering for empty strings is only possible after type in empty space and delete it again, then a list of all elements is displayed for selection including empty fields.

## Acceptance Criteria
- [ ] Edge cases of the autocomplete have been resolved

## Out of Scope

- ...

No files or tests are named. Start by tracing the autocomplete flow in the frontend and backend, focusing on enum filtering, selection state, searches for selected items, and empty-string searches. Done means all listed edge cases are resolved and the acceptance criterion is verifiable; identify the relevant test coverage before changing behavior.

Written by the indexing model from the issue text.