fix: sanitize tab chars and avoid double .md extension in manual download filename

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/application/service/knowledge.go

@@ -6752,12 +6752,15 @@ func ensureManualFileName(title string) string {
 // back to "untitled".
 func sanitizeManualDownloadFilename(title string) string {
 	safeName := strings.NewReplacer(
-		"\n", "", "\r", "", "/", "-", "\\", "-", "\"", "'",
+		"\n", "", "\r", "", "\t", "", "/", "-", "\\", "-", "\"", "'",
 	).Replace(title)
 	if strings.TrimSpace(safeName) == "" {
 		safeName = "untitled"
 	}
-	return safeName + manualFileExtension
+	if !strings.HasSuffix(strings.ToLower(safeName), manualFileExtension) {
+		safeName += manualFileExtension
+	}
+	return safeName
 }
 
 func (s *knowledgeService) triggerManualProcessing(ctx context.Context,

internal/application/service/knowledge_manual_test.go

@@ -72,6 +72,16 @@ func TestSanitizeManualDownloadFilename(t *testing.T) {
 			title: "知识库文章",
 			want:  "知识库文章.md",
 		},
+		{
+			name:  "tab character stripped",
+			title: "file\tname",
+			want:  "filename.md",
+		},
+		{
+			name:  "title already ending in .md not double-extended",
+			title: "guide.md",
+			want:  "guide.md",
+		},
 	}
 
 	for _, tt := range tests {