fix: retain knowledge indexing errors and explain failure causes

internal/handler/knowledge_index_error_test.go

@@ -0,0 +1,20 @@
+package handler
+
+import (
+	"testing"
+	"time"
+)
+
+func TestKnowledgeIndexFailureRemainsVisibleUntilCleared(t *testing.T) {
+	occurred := time.Now().Add(-time.Hour)
+	status := map[string]interface{}{}
+	appendKnowledgeIndexError(status, "test-only embedding failure: 401 Unauthorized", occurred)
+	if status["last_error"] == nil || status["last_error_time"] != occurred.Format(time.RFC3339) {
+		t.Fatal("older failure disappeared")
+	}
+	cleared := map[string]interface{}{}
+	appendKnowledgeIndexError(cleared, "", time.Time{})
+	if _, ok := cleared["last_error"]; ok {
+		t.Fatal("cleared failure remained")
+	}
+}