Optimize CLI and console annotation subsystems

[Piyush0049]

Optimize CLI and console annotation subsystems

This pull request introduces targeted performance optimizations and concurrency fixes to the CLI and console annotation subsystems.

Key improvements:

• Resolves race conditions in WebSocketSession by introducing volatile visibility and strict null guards.
• Caches Locales in ServerSideImpl using a static ConcurrentHashMap to eliminate GC churn and speed up CLI boot times.
• Prevents Mojibake character corruption via explicit Charset propagation in ConsoleAnnotationOutputStream.

Testing done

Automated testing was performed locally via Maven to ensure strict backwards compatibility and structural soundness. The following targeted tests were executed and passed with zero failures:

• CLIActionTest
• Security3630Test

If maintainers require further test coverage or specific structural adjustments, please feel free to convert this PR into a draft while those additional items are resolved.

Screenshots (UI changes only)

Before

N/A

After

N/A

Proposed changelog entries

• Prevent NullPointerExceptions in WebSocket CLI sessions due to thread race conditions.
• Improve CLI connection performance by caching available Locales.
• Prevent character corruption in console annotations by explicitly honoring stream Charsets.

Proposed changelog category

/label bug, rfe

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.
• New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadocs, as appropriate.
• New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable.
• 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).
• For dependency updates, there are links to external changelogs and, if possible, full differentials.
• For new APIs and extension points, there is a link to at least one consumer.

Desired reviewers

@jenkinsci/core-pr-reviewers

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.
• Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
• Proper changelog labels are set so that the changelog can be generated automatically.
• 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).
• If it would make sense to backport the change to LTS, be a Bug or Improvement, and either the issue or pull request must be labeled as lts-candidate to be considered.

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