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/actions.go

@@ -7,7 +7,6 @@ import (
 	"io"
 	"log/slog"
 	"net/http"
-	"reflect"
 
 	"git.golem15.com/golem15/summercms/modules/bouncer"
 	"git.golem15.com/golem15/summercms/modules/pact"
@@ -219,25 +218,18 @@ func onlyFillScalars(fill []string, values map[string]any) map[string]any {
 }
 
 // isJSONScalar reports whether v encodes as a JSON string, number, boolean or
-// null.
+// null. It decides by encoding the value, not by its reflect.Kind: a named
+// scalar whose MarshalJSON writes an array or object is not a scalar, and a
+// value that cannot be encoded (NaN, an infinity) is dropped rather than
+// failing the whole response.
 func isJSONScalar(v any) bool {
-	if v == nil || nestedValue(v) {
-		return v == nil
-	}
-	rv := reflect.ValueOf(v)
-	for rv.Kind() == reflect.Pointer {
-		if rv.IsNil() {
-			return true
-		}
-		rv = rv.Elem()
-	}
-	switch rv.Kind() {
-	case reflect.Bool, reflect.String,
-		reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
-		reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64,
-		reflect.Float32, reflect.Float64:
-		return true
-	default:
+	raw, err := json.Marshal(v)
+	if err != nil || len(raw) == 0 {
 		return false
 	}
+	switch raw[0] {
+	case '{', '[':
+		return false
+	}
+	return true
 }