Fix main_class displaying extra arguments on Arthas startup

[Copilot]

sun.java.command contains the full launch command — main class name followed by all application arguments — but was being used as-is for main_class, producing output like:

main_class   org.springframework.boot.loader.JarLauncher --server.port=7001

Change

• PidUtils.java: Extract only the first token from sun.java.command (split on first space), so mainClass() returns just the class/jar name without any trailing arguments.

String command = System.getProperty("sun.java.command", "");
int spaceIndex = command.indexOf(' ');
MAIN_CLASS = spaceIndex != -1 ? command.substring(0, spaceIndex) : command;

Original prompt

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This section details on the original issue you should resolve

<issue_title>arthas 启动时，获取的 main class 打印信息有多余部分</issue_title>
<issue_description>main_class org.springframework.boot.loader.JarLauncher --server.port=7001</issue_description>

<agent_instructions>排查一下为什么 arthas 启动时打印的 main_class 有可能有多余的信息</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes arthas 启动时，获取的 main class 打印信息有多余部分 #3148

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