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/cli/fetch/once.go

@@ -2,6 +2,7 @@ package fetch
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"log/slog"
 	"slices"
@@ -73,6 +74,9 @@ func commandOnce(opts *cli.Options) *cobra.Command {
 				return sinceErr
 			}
 
+			if conf.Sources.FindATender.APIRoot == "" {
+				return errors.New("sources.find_a_tender.api_root is not set")
+			}
 			findATender := findatender.New(conf, conf.Sources.FindATender.APIRoot)
 			defer findATender.Shutdown()