Expose ruby_plugin on filter/output delegators

[kaisecheng]

What does this PR do?

Add a ruby_plugin JRuby method to all filter and output delegator types so that callers can access the underlying Ruby plugin instance when inspecting plugin config or instance variables.

Ruby-backed delegators (FilterDelegatorExt, OutputDelegatorExt, OutputStrategyExt) return the inner plugin object. Java-native delegators (JavaFilterDelegatorExt, JavaOutputDelegatorExt) return nil because they use the Java config system rather than LogStash::Config::Mixin.

Why is it important/What is the impact to the user?

Filters and outputs are wrapped in Java delegators whose ancestor chain is the Java class hierarchy. Code that needs to inspect plugin config (e.g. reading SSL certificate paths for hot-reload) must unwrap the delegator first to access get_config and instance variables. This PR provides a standard unwrapping method across all delegator types.

Checklist

• My code follows the style guidelines of this project
• I have added tests that prove my fix is effective or that my feature works
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files

How to test this PR locally

./gradlew :logstash-core:rubyTests --tests "org.logstash.config.ir.compiler.OutputDelegatorTest" --rerun-tasks

Related issues

• Part of TLS hot-reload feature

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @kaisecheng? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label

[elasticmachine]

💛 Build succeeded, but was flaky

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• Commit: 1ec1e8b

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