Optimize JnlpAgentReceiver to use O(1) cache lookup

[aryawadhwa]

This PR optimizes the node lookup in JnlpAgentReceiver for inbound agents. It changes an $O(N)$ linear search over all nodes to an $O(1)$ lookup using Jenkins.get().getComputer(clientName). This improves performance when a large number of nodes are connected.

This is a follow-up optimization to the inbound agent secret rotation changes.

Testing done

• Tested with mvn test in the jenkins/slaves module to ensure no regressions in JNLP receiver logic.
• Interactive Testing Note: As requested by reviewers, this should be tested interactively on a local Jenkins instance with multiple inbound agents to ensure agent connections are properly verified and secrets are matched correctly. (I am running this locally to verify before marking ready).

Proposed changelog entries

• Optimize JNLP agent connection lookup to use O(1) computer cache instead of O(N) node iteration

Proposed changelog category

/label 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.
• 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.
• 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

@MarkEWaite

[MarkEWaite]

I'm not persuaded that this is valuable enough to justify the added complexity. Even with hundreds of nodes, the lookup time will be indistinguishable between a linear lookup and a hashed lookup.

The pull request includes unrelated changes that must be removed. It is probably best to raise an issue in the issue tracker and describe the problem that you're trying to solve. Provide examples of the measurements that you've used to confirm there is a problem.

Closing because it includes unrelated changes and needs more discussion before it is implemented.