Migrate from Commons Lang to native Java Platform functionality

[timja]

No need to use a third-party library when this functionality is available in the Java Platform.

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

• StringUtils.isEmpty / isNotEmpty / isBlank / isNotBlank → the hudson.Util.fixEmpty* helpers.
• ArrayUtils.contains(arguments, x) → Arrays.asList(arguments).contains(x) in
  JenkinsBrowserStackLocalTest.
• RandomStringUtils.random(48, letters, numbers) in Tools.getUniqueString is now a small local
  helper over SecureRandom building from the same letter/digit pools. Commons Lang 2 backed that
  call with a plain java.util.Random; since the result is used as a tracking identifier I did not
  want to hand-write a weaker generator. One deliberate narrowing: Commons Lang 2 would draw from the
  full character range if both flags were false, whereas this version assumes at least one is set.
  Both call sites pass (true, true).
• Removed an import of io.jenkins.cli.shaded.org.apache.commons.lang.StringUtils in
  QualityDashboardPipelineTracker. That is the copy of Commons Lang shaded inside the Jenkins CLI
  jar — almost certainly an IDE auto-import accident, and not something to depend on. It had no call
  sites left after the rewrite.

No new dependency is added — commons-lang3-api requires core 2.479.3 and this plugin's baseline is
2.334.

Testing done

mvn -B -ntp clean verify on Java 11 / macOS: 38 tests, 6 errors — and unmodified master gives
exactly the same 38 tests and 6 errors, so they are pre-existing and unrelated to this change. Five
are HtmlUnit failing on modern JS (Cannot find function createHTMLDocument) and one is ByteBuddy
being unable to self-attach (Can not attach to current VM).

Newer JDKs do not work at all here: maven-license-plugin:1.7 throws ExceptionInInitializerError
on Java 17 and 21, again on unmodified master.

The ban-commons-lang-2 enforcer rule was left disabled because it needs parent POM
6.2116.v7501b_67dc517 or newer; bumping the parent from 3.4 was out of scope here.

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
@parameter.