autoresearch iter 8: render synthesized notification content in stable order

bodyContent's fallback ranged over the body map, and Go randomizes map
iteration, so the same notification rendered its fields in a different order
on every send. Observed failing before the fix: the second call already
reordered the output. Iterate sorted keys instead.

backend/plugins/domain/message_gateway/push/template.go

@@ -6,6 +6,8 @@ package push
 import (
 	"encoding/json"
 	"fmt"
+	"maps"
+	"slices"
 	"strconv"
 	"strings"
 )
@@ -87,13 +89,15 @@ func bodyTitle(body map[string]any) string {
 
 // bodyContent returns the notification body, rendering every entry with format
 // (a "%s … %v" pair) and joining them with sep when no content field is given.
+// Entries render in sorted key order so identical bodies always produce
+// identical text.
 func bodyContent(body map[string]any, format, sep string) string {
 	if c, ok := body["content"].(string); ok && c != "" {
 		return c
 	}
 	parts := make([]string, 0, len(body))
-	for k, v := range body {
-		parts = append(parts, fmt.Sprintf(format, k, v))
+	for _, k := range slices.Sorted(maps.Keys(body)) {
+		parts = append(parts, fmt.Sprintf(format, k, body[k]))
 	}
 	return strings.Join(parts, sep)
 }

backend/plugins/domain/message_gateway/push/template_test.go

@@ -73,3 +73,24 @@ func TestParseTemplate(t *testing.T) {
 		})
 	}
 }
+
+// Synthesized content must render in a stable order, otherwise two identical
+// notifications produce different text on every send.
+func TestBodyContentFallbackIsDeterministic(t *testing.T) {
+	body := map[string]any{
+		"zebra":   1,
+		"alpha":   2,
+		"mike":    3,
+		"charlie": 4,
+		"yankee":  5,
+	}
+
+	first := bodyContent(body, "%s=%v", ",")
+	for i := 1; i <= 50; i++ {
+		if got := bodyContent(body, "%s=%v", ","); got != first {
+			t.Fatalf("bodyContent order changed on call %d: %q != %q", i, got, first)
+		}
+	}
+
+	assert.Equal(t, "alpha=2,charlie=4,mike=3,yankee=5,zebra=1", first)
+}