Fixed string-items-list autocomplete selection and blur handling

[vvlladd28]

Summary

• Replace manual addOnBlur handler with [matChipInputAddOnBlur]="true" directive and dedicated onOptionSelected handler for proper autocomplete selection
• Add (mousedown)="$event.preventDefault()" on mat-options to prevent premature blur closing the panel before selection
• Conditionally hide "not found" option when allowUserValue is enabled
• Extend matAutocompleteDisabled check to also consider fetchOptionsFn
• Simplify ngOnInit value change handling and improve clear() method reset logic

Test plan

• Verify autocomplete dropdown stays open when clicking options
• Verify chip input add-on-blur works correctly with the directive
• Verify custom user values can be entered when allowUserValue is true
• Verify "not found" message is hidden when allowUserValue is enabled
• Verify fetchOptionsFn properly enables autocomplete