Log Seedream payload shape before forwarding to the gateway

Seedream image jobs now log whether image input was present, whether it was sent as a single string or an array, how many references were included, and which size preset was used. This makes it possible to prove whether missing reference images are a server bug or a downstream gateway issue.

Constraint: The test server needed a low-noise way to confirm what reached /v1/images/generations without changing the external API contract
Rejected: Rely on gateway-side observation alone | it could not distinguish whether the image array was dropped before or after the server boundary
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep this Seedream request-shape log until the gateway team has finished validating image-array forwarding end to end
Tested: go test ./internal/server
Tested: Seedream 4.5 job on test server with three image refs after deploying binary (job_47513ead2708)
Not-tested: Long-term log volume impact under sustained Seedream traffic

internal/server/newapi.go

@@ -615,6 +615,14 @@ func (c *newAPIClient) generateSeedreamImageRefs(ctx context.Context, token, mod
 		}
 		payload["image"] = imageInput
 	}
+	log.Printf(
+		"popiartServer: Seedream image request model=%s has_image=%t image_type=%s image_count=%d size=%v",
+		modelID,
+		payload["image"] != nil,
+		describeImagePayloadType(payload["image"]),
+		countImagePayloadItems(payload["image"]),
+		payload["size"],
+	)
 
 	body, err := json.Marshal(payload)
 	if err != nil {
@@ -1340,6 +1348,39 @@ func encodeSeedreamReferenceInputs(refs []imageEditReference) (any, error) {
 	return encodeImageReferenceInputs(refs)
 }
 
+func describeImagePayloadType(value any) string {
+	switch value.(type) {
+	case nil:
+		return "missing"
+	case string:
+		return "string"
+	case []string:
+		return "string_array"
+	case []any:
+		return "array"
+	default:
+		return fmt.Sprintf("%T", value)
+	}
+}
+
+func countImagePayloadItems(value any) int {
+	switch typed := value.(type) {
+	case nil:
+		return 0
+	case string:
+		if strings.TrimSpace(typed) == "" {
+			return 0
+		}
+		return 1
+	case []string:
+		return len(typed)
+	case []any:
+		return len(typed)
+	default:
+		return 1
+	}
+}
+
 func encodeImageReference(ref imageEditReference) (string, error) {
 	if rawURL := strings.TrimSpace(ref.URL); rawURL != "" {
 		return rawURL, nil