An empty api_root now fails at start, for an enabled find_a_tender or fts_mock source and for fetch once

An enabled Find a Tender or mock source with no api_root used to start
and then fail on every fetch. Its worker now returns an error naming the
missing key before it starts, which stops Run. "fetch once" checks
sources.find_a_tender.api_root the same way. TestRunMissingAPIRoot covers
both sources, and TestRunLogsSourceErrors sets an api_root so it still
tests the zero interval. The section docs and internal/config/README.md
say what an empty api_root means.

internal/fetcher/worker_ftsmock.go

@@ -20,6 +20,10 @@ func runFTSMock(
 	conf *config.JobSearchConfig,
 	since time.Time,
 ) error {
+	if conf.Sources.FTSMock.APIRoot == "" {
+		return errors.New("fetcher: sources.fts_mock.api_root is not set")
+	}
+
 	worker := util.Worker{
 		Logger:   tagged(logger),
 		Name:     "FTSMock",