refactor(log):优化日志读取逻辑并修复潜在问题

- 使用 StringBuilder 替代 StringBuffer 提升性能- 采用 StandardCharsets.UTF_8 确保字符编码一致性
-优化行号判断逻辑，提升代码可读性- 移除冗余注释和无用代码片段-修复日志文件读取时的行数跳过逻辑
- 简化 LogResult 构造逻辑，移除不必要的注释块

xxl-job-core/src/main/java/com/xxl/job/core/context/XxlJobHelper.java

@@ -160,7 +160,7 @@ public class XxlJobHelper {
         // appendlog
         String logFileName = xxlJobContext.getJobLogFileName();
 
-        if (logFileName!=null && logFileName.trim().length()>0) {
+        if (logFileName!=null && !logFileName.trim().isEmpty()) {
             XxlJobFileAppender.appendLog(logFileName, formatAppendLog);
             return true;
         } else {

xxl-job-core/src/main/java/com/xxl/job/core/log/XxlJobFileAppender.java

@@ -84,17 +84,16 @@ public class XxlJobFileAppender {
 	/**
 	 * append log
 	 *
-	 * @param logFileName
+	 * @param logFileName	log file name
-	 * @param appendLog
+	 * @param appendLog		append log
 	 */
 	public static void appendLog(String logFileName, String appendLog) {
 
 		// log file
-		if (logFileName==null || logFileName.trim().length()==0) {
+		if (logFileName==null || logFileName.trim().isEmpty()) {
 			return;
 		}
 		File logFile = new File(logFileName);
-
 		if (!logFile.exists()) {
 			try {
 				logFile.createNewFile();
@@ -111,29 +110,20 @@ public class XxlJobFileAppender {
 		appendLog += "\r\n";
 		
 		// append file content
-		FileOutputStream fos = null;
+        try (FileOutputStream fos = new FileOutputStream(logFile, true)) {
-		try {
+            fos.write(appendLog.getBytes(StandardCharsets.UTF_8));
-			fos = new FileOutputStream(logFile, true);
+            fos.flush();
-			fos.write(appendLog.getBytes("utf-8"));
+        } catch (Exception e) {
-			fos.flush();
+            logger.error(e.getMessage(), e);
-		} catch (Exception e) {
+        }
-			logger.error(e.getMessage(), e);
-		} finally {
-			if (fos != null) {
-				try {
-					fos.close();
-				} catch (IOException e) {
-					logger.error(e.getMessage(), e);
-				}
-			}
-		}
 		
 	}
 
 	/**
 	 * support read log-file
 	 *
-	 * @param logFileName
+	 * @param logFileName	log file name
+	 * @param fromLineNum	from line num
 	 * @return log content
 	 */
 	public static LogResult readLog(String logFileName, int fromLineNum){