Remove usages of Commons Lang 2

[timja]

No need to depend on a third-party library here when the Java Platform provides this
functionality natively.

Part of the effort to remove Commons Lang 2 from Jenkins core — jenkinsci/jenkins#16404,
jenkinsci/jenkins#26105. Commons Lang 2 is EOL and carries an unfixed advisory
(GHSA-j288-q9x7-2f5v).

What's changed

Rewritten in plain Java — this plugin has no BOM and an org.jvnet.hudson.plugins:plugin:1.395
parent, so neither commons-lang3-api nor a direct commons-lang3 dependency is usable.

• MutableInt (a mutable counter in maps) becomes java.util.concurrent.atomic.AtomicInteger.
• ToStringBuilder.reflectionToString(this, DEFAULT_STYLE) becomes explicit toString()
  implementations listing the same fields. The output format changes slightly (it is only used for
  logging), but no longer depends on reflection.
• StringUtils.isEmpty / isNotEmpty become plain null plus length() checks.

Testing done

mvn -B -ntp clean verify fails locally in maven-hpi-plugin:1.57:apt-compile, which cannot run
on a modern JDK — reproducible on an unmodified checkout.

The ban-commons-lang-2 enforcer rule was left disabled: the parent POM predates the rule entirely.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

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🤖 This pull request was generated with AI assistance (Claude Code) as part of a bulk migration
across Jenkins plugins. If anything here looks wrong, please comment on this PR or contact @timja.