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/fetcher_test.go

@@ -229,7 +229,9 @@ func TestRunLogsSourceErrors(t *testing.T) {
 	// A zero interval makes each worker fail straight away.
 	conf := &config.JobSearchConfig{}
 	conf.Sources.FindATender.Enabled = true
+	conf.Sources.FindATender.APIRoot = "http://127.0.0.1:1"
 	conf.Sources.FTSMock.Enabled = true
+	conf.Sources.FTSMock.APIRoot = "http://127.0.0.1:1"
 	conf.Sources.Sleeper.Enabled = true
 	err := Run(context.Background(), conf, time.Time{})
 
@@ -248,6 +250,37 @@ func TestRunLogsSourceErrors(t *testing.T) {
 	}
 }
 
+// An enabled Find a Tender or mock source with no api_root fails straight
+// away, naming the setting.
+func TestRunMissingAPIRoot(t *testing.T) {
+	quiet := slog.New(slog.DiscardHandler)
+	tests := []struct {
+		name   string
+		enable func(*config.JobSearchConfig)
+		key    string
+	}{
+		{"find_a_tender", func(conf *config.JobSearchConfig) {
+			conf.Sources.FindATender.Enabled = true
+			conf.Sources.FindATender.Interval = time.Hour
+		}, "sources.find_a_tender.api_root"},
+		{"fts_mock", func(conf *config.JobSearchConfig) {
+			conf.Sources.FTSMock.Enabled = true
+			conf.Sources.FTSMock.Interval = time.Hour
+		}, "sources.fts_mock.api_root"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			conf := &config.JobSearchConfig{}
+			tt.enable(conf)
+
+			err := Run(context.Background(), conf, time.Time{}, WithLogger(quiet))
+			if err == nil || !strings.Contains(err.Error(), tt.key) {
+				t.Errorf("Run = %v; want an error naming %s", err, tt.key)
+			}
+		})
+	}
+}
+
 func TestRunNoSourcesEnabled(t *testing.T) {
 	done := make(chan struct{})
 	go func() {

internal/fetcher/worker_findatender.go

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

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",