The back() handler had the same filtered-index drift bug as the Enter
and Ctrl+D transitions: when the user presses Esc to clear an active
filter on the provider stage, providerIdx was reset to 0, losing the
highlighted provider. Apply the same providerIndexAfterClearingFilter
fix as the other three transition paths.
Also adds edge-case tests for the helper: undefined provider, slug not
found, empty rows, and duplicate slug first-match behavior.
Found by hermes-pr-review Phase 2 + hermes-agent-dev 3-agent review.
58919f68ab