Improve diagnostics for external agent attachment

[codingkiddo]

Summary

Improves diagnostics for external Byte Buddy agent attachment.

When net.bytebuddy.agent.attacher.dump is configured, the generated external attachment command line is appended to the configured dump file before the external attacher process is started.

Motivation

Issue #1602 mentions that external attachment failures can be difficult to troubleshoot because users only see the final failure message. Including the generated command line in the existing dump output makes it easier to inspect the Java executable, classpath, attacher class, target VM type, process id, agent path, and native flag used for the external attachment attempt.

Changes

• Refactors the external attachment process command into a List<String>
• Appends the generated command line to the configured dump file
• Keeps normal behavior unchanged when net.bytebuddy.agent.attacher.dump is not configured
• Leaves the existing external process invocation behavior unchanged

Scope

This PR intentionally does not add support for extra JVM arguments or change attachment behavior. Those parts may require a separate design discussion.

Addresses part of #1602.

Testing

• ./mvnw -pl byte-buddy-agent -am test
• ./mvnw -pl byte-buddy-agent -am -DskipTests package
• Ran a local reflection-based smoke test to invoke the external attachment path with net.bytebuddy.agent.attacher.dump=/tmp/byte-buddy-attacher.dump
• Verified the dump file contains the generated external attachment command line before the attacher failure details
• Verified an invalid dump path does not introduce a new failure in the external attachment flow
• Verified no dump file is created when net.bytebuddy.agent.attacher.dump is not configured

Example verified dump content:

Byte Buddy external attachment command line:
/Library/Java/JavaVirtualMachines/jdk-1.8.jdk/Contents/Home/jre/bin/java
-Dnet.bytebuddy.agent.attacher.dump=/tmp/byte-buddy-attacher.dump
-cp
...
net.bytebuddy.agent.Attacher
com.sun.tools.attach.VirtualMachine
...
false