VSN-2946/feat(driver/rh): add searchable responsable (manager) dropdowns across forms

Add three optional searchable "Responsable Exploitation / Flotte / RH" dropdowns to the chauffeur, salarié and candidature forms, populated from BO collaborators via autocomplete and stored as UUID + cached name. Introduce a shared useResponsableOptions hook and expose the collaborator uuid on the autocomplete store. Reset ComboboxField's server-search list on open/close so a previous query no longer lingers. Fix the salarié tab showing "undefined undefined" and empty fields after save by returning the employee→candidat shape to the stay-on-tab flow.