fix: harden multi-pod delivery and load verification

loadtest/im-pressure/collector_test.go

@@ -0,0 +1,31 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+func TestDetectCollectorPhase(t *testing.T) {
+	start := StartInfo{RunID: "run1", StartAt: time.Now().Add(-time.Second)}
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		_ = json.NewEncoder(w).Encode(map[string]any{"start": start})
+	}))
+	defer server.Close()
+	client := &http.Client{Timeout: time.Second}
+	if got := detectCollectorPhase(context.Background(), client, "before", server.URL, ""); got != "during" {
+		t.Fatalf("phase=%s", got)
+	}
+	output := filepath.Join(t.TempDir(), "coordinator.json")
+	if err := os.WriteFile(output, []byte("{}"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if got := detectCollectorPhase(context.Background(), client, "during", server.URL, output); got != "after" {
+		t.Fatalf("phase=%s", got)
+	}
+}