H-116: close visitor payload trust boundaries (#19)

* H-116: close visitor payload trust boundaries

* H-129: unblock SQLite backend tests

* H-129: remove stale last-seen response assertions

---------

Co-authored-by: Rogee <rogee@ipao.vip>

backend/internal/handler/widget/widget_handler_test.go

@@ -50,6 +50,29 @@ type recordingWidgetEventPublisher struct {
 	payload     interface{}
 }
 
+func TestPublicMessagePayloadSanitizesIdentity(t *testing.T) {
+	senderID := uint(9)
+	message := model.Message{
+		SenderID:          &senderID,
+		SenderType:        string(model.SenderTypeAgentBot),
+		Content:           "same reply",
+		ContentAttributes: datatypes.JSON(`[{"business":"keep","nested":{"sender_name":"Captain","ai_takeover_active":true}}]`),
+	}
+
+	publicPayload := publicMessagePayload(message, model.Conversation{Base: model.Base{ID: 1}})
+	assert.NotContains(t, publicPayload, "sender")
+	assert.Equal(t, "same reply", publicPayload["content"])
+	assert.Equal(t, []any{map[string]any{"business": "keep", "nested": map[string]any{}}}, publicPayload["content_attributes"])
+	message.SenderType = string(model.SenderTypeUser)
+	assert.Equal(t, publicPayload, publicMessagePayload(message, model.Conversation{Base: model.Base{ID: 1}}))
+
+	widgetPayload := widgetMessagePayload(message, 1)
+	assert.Equal(t, publicPayload["content_attributes"], widgetPayload["content_attributes"])
+
+	message.ContentAttributes = datatypes.JSON(`{"sender_name":`)
+	assert.Empty(t, publicMessagePayload(message, model.Conversation{})["content_attributes"])
+}
+
 func (p *recordingWidgetEventPublisher) PublishWidgetEvent(accountID uint, pubsubToken string, eventType string, payload interface{}) {
 	p.accountID = accountID
 	p.pubsubToken = pubsubToken