Fixes #26285

The administrative monitor configuration page was serializing am.id from Groovy. For DoubleLaunchChecker, Groovy resolves that property through getId(), which returns the current process ID instead of the stable AdministrativeMonitor.id field. Saving the global configuration then disables the monitor because the submitted value does not match the persisted monitor ID.

This uses direct field access for the serialized monitor ID and adds a regression test for the global configuration round-trip.

Testing done

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64 PATH=/usr/lib/jvm/java-21-openjdk-amd64/bin:$PATH /tmp/apache-maven-3.9.6/bin/mvn -pl test -Dtest=jenkins.management.AdministrativeMonitorsConfigurationTest test

Also installed the required local jenkins-core and jenkins-war snapshots first with the documented quick-build profile because this environment only had Maven 3.8.7 on PATH:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64 PATH=/usr/lib/jvm/java-21-openjdk-amd64/bin:$PATH /tmp/apache-maven-3.9.6/bin/mvn '-Pquick-build,!yarn-execution,!yarn-lint,!yarn-ci-lint' -pl war,bom -am -rf :jenkins-core -DskipTests -Dspotbugs.skip=true -Dcheckstyle.skip=true -Dspotless.check.skip=true install

Screenshots (UI changes only)

N/A

Before

After

Proposed changelog entries

• Keep the Duplicate Jenkins checker enabled when saving the global configuration (regression in 2.549).

Proposed changelog category

/label bug

Proposed upgrade guidelines

N/A

Submitter checklist

• The issue, if it exists, is well-described.
• The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples). Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
• There is automated testing or an explanation as to why this change has no tests.
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• UI changes do not introduce regressions when enforcing the current default rules of Content Security Policy Plugin. In particular, new or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content Security Policy (CSP) directives (see documentation).
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Desired reviewers

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Before the changes are marked as ready-for-merge:

Maintainer checklist

• There are at least two (2) approvals for the pull request and no outstanding requests for change.
• Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
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• If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
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