fix(10.1): WR-04 judge action fill values by their JSON encoding

isJSONScalar now encodes each value and keeps it only when the encoding
is a string, number, boolean or null, so a named scalar whose MarshalJSON
writes an array or object, NaN and the infinities are dropped. writeJSON
encodes into a buffer before the status line, so an encode failure is a
logged 500 with the generic envelope instead of a 200 with a truncated
body.

modules/cabana/contracts.go

@@ -1,7 +1,9 @@
 package cabana
 
 import (
+	"bytes"
 	"encoding/json"
+	"log/slog"
 	"net/http"
 	"strings"
 	"time"
@@ -187,8 +189,20 @@ func WriteErrorDetails(w http.ResponseWriter, status int, code, message string,
 	})
 }
 
+// writeJSON encodes body before the status line goes out, so a value that
+// cannot be encoded (NaN, an infinity, a failing MarshalJSON) is a logged 500
+// with the generic body instead of a 200 with a truncated one.
 func writeJSON(w http.ResponseWriter, status int, body any) {
+	var buf bytes.Buffer
+	if err := json.NewEncoder(&buf).Encode(body); err != nil {
+		slog.Error("cabana: response could not be encoded", "status", status, "error", err)
+		buf.Reset()
+		_ = json.NewEncoder(&buf).Encode(map[string]any{
+			"error": map[string]any{"code": "error", "message": msgServerError, "details": map[string]any{}},
+		})
+		status = http.StatusInternalServerError
+	}
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(status)
-	_ = json.NewEncoder(w).Encode(body)
+	_, _ = w.Write(buf.Bytes())
 }