Update Robot Framework test suite and JSON payloads for AppContext API...

Update Robot Framework test suite and JSON payloads for AppContext API validation- Refactored SysUeAppsContext.robot test cases and keywords for correct contextId handling and negative test coverage.- Updated AppContext.json and UpdateAppContext.json to use contextId placeholder for dynamic replacement.- Added invalid parameter to UpdateAppContext_BR.json for 400 Bad Request testing.- Adjusted AppContext_BR.json and environment/variables.txt for improved test consistency.