- discard an in-flight discovery when the runtime switches, mirroring the
isRequestCurrent pattern, so a late completion cannot repopulate the
cleared map and suppress a fresh scan
- treat a 501 from /api/fs/git-dirs as an explicit 'unsupported' marker
instead of a generic failure; the VS Code webview now answers the route
with unsupportedWebRouteResponse so non-repo roots show the honest
not-a-repository state without a futile Retry