client: fix JAVA_HOME NPE at startup and malformed cmdQueueLimit token

[jutur]

Main.getJavaVersion() ran inside the class's static initializer and called Paths.get(System.getenv("JAVA_HOME"), "release") unguarded. When JAVA_HOME isn't set, System.getenv() returns null and Paths.get(null, ..) throws NullPointerException -- only IOException was caught, so the NPE escaped as ExceptionInInitializerError and killed the CLI before it could start. Falls back to the java.home system property (the JVM the client itself is running on) when JAVA_HOME is unset or blank, and returns null (unknown version) only if neither is available.

Separately, Client.java built the --cmdQueueLimit agent argument as ",=" + Args.CMD_QUEUE_LIMIT + cmdQueueLimit instead of the "," + KEY + "=" + value pattern every other agent arg in this method follows. The leading "=" with no key meant the agent's parser saw a junk empty-key entry rather than a CMD_QUEUE_LIMIT value, so the setting was silently dropped.

Fixes #887, #886

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• I identified the affected module(s) and kept unrelated changes out of this PR.
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# GRADLE_USER_HOME=$(pwd)/.gradle-user ./gradlew :module:test

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