refactor: 简化词法异常错误信息

- 移除了错误信息中的"词法错误:"前缀，使信息更加简洁
- 保留了非法字符序列和位置信息，确保错误定位准确

src/main/java/org/jcnc/snow/compiler/lexer/scanners/UnknownTokenScanner.java

@@ -50,6 +50,6 @@ public class UnknownTokenScanner extends AbstractTokenScanner {
         if (lexeme.isEmpty())
             lexeme = String.valueOf(ctx.advance());
         // 抛出词法异常，并带上错误片段与具体位置
-        throw new LexicalException("词法错误: 非法字符序列 '" + lexeme + "'", line, col);
+        throw new LexicalException("非法字符序列 '" + lexeme + "'", line, col);
     }
 }