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/phase101_actions_test.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"io/fs"
+	"math"
 	"net/http"
 	"net/http/httptest"
 	"os"
@@ -52,6 +53,11 @@ type actGroup struct {
 
 func (actGroup) TableName() string { return "cabana_ext_groups" }
 
+// actTags is a string kind that encodes as a JSON array.
+type actTags string
+
+func (t actTags) MarshalJSON() ([]byte, error) { return json.Marshal(strings.Split(string(t), ",")) }
+
 // actSpy records the inputs the registered actions receive.
 type actSpy struct {
 	mu    sync.Mutex
@@ -136,6 +142,9 @@ func (c actController) AdminActions() []pact.AdminAction {
 				return pact.AdminActionResult{}, errors.New("upstream said hunter2")
 			case "nested":
 				return pact.AdminActionResult{Fill: map[string]any{"name": []string{"a"}, "active": false}}, nil
+			case "encoded":
+				// Scalar kinds that do not encode as JSON scalars (WR-04).
+				return pact.AdminActionResult{Fill: map[string]any{"name": actTags("a,b"), "active": math.NaN()}}, nil
 			}
 			return pact.AdminActionResult{
 				Message: "acme.demo::lang.gadgets.looked_up",
@@ -361,6 +370,14 @@ func TestPhase101Actions(t *testing.T) {
 		}
 	})
 
+	t.Run("fill is judged by its JSON encoding, not its kind", func(t *testing.T) {
+		rec := env.expect(t, http.StatusOK, http.MethodPost, widget, `{"values":{"name":"encoded"}}`, "bearer")
+		if result := actResult(t, rec); !reflect.DeepEqual(result.Fill, map[string]any{}) {
+			t.Fatalf("fill = %#v", result.Fill)
+		}
+		env.spy.take()
+	})
+
 	t.Run("widget on the create form gets no record", func(t *testing.T) {
 		env.expect(t, http.StatusOK, http.MethodPost, widget, `{}`, "bearer")
 		calls := env.spy.take()