fix: removeCodeBlocks 给每行末尾追加 \n 导致发送消息多一个换行

改为用 strings.Join 拼接，只在行间加 \n，末行不追加。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/platform-bridge/internal/adapter/qq/adapter.go

@@ -594,20 +594,17 @@ func removeHeadings(s string) string {
 }
 
 func removeCodeBlocks(s string) string {
-	result := ""
+	var kept []string
 	inCode := false
 	for _, line := range splitLines(s) {
 		if hasPrefix(line, "```") {
 			inCode = !inCode
 			continue
 		}
-		if inCode {
-			result += line + "\n"
-		} else {
-			result += line + "\n"
-		}
+		kept = append(kept, line)
 	}
-	return result
+	_ = inCode
+	return strings.Join(kept, "\n")
 }
 
 func splitLines(s string) []string {