feat(complete): Add ValueCompleter::complete at for indexed multi-value completion

feat(complete): Add ValueCompleter::complete at for indexed multi-value completion: a task in repo2rlenv-pr-diff (Harbor dataset). Title: feat(complete): Add ValueCompleter::complete at for indexed multi-value completion

The task

**Title:** feat(complete): Add ValueCompleter::complete_at for indexed multi-value completion

Part of FineEnvs/repo2rlenv-pr-diff.