Avoid Guice circular issue and implement test to ensure extension is activated

[jonesbusy]

Testing done

Automated tests

### Submitter checklist
- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- [ ] Link to relevant issues in GitHub or Jira
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- [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[jonesbusy]

Can you take a look @sridamul

[sridamul]

Can you also remove the validation? or need to figure out other way to display the message correctly.

[sridamul]

Can we use the extension from the extension Indexer to validate class? If possible, we can use it after the completion of this GSoC'24 project

[jonesbusy]

Can you also remove the validation? or need to figure out other way to display the message correctly.

https://github.com/jenkinsci/extension-filter-plugin/pull/70

By reading doc this can be achieved using an other class loader to check class existence

[jonesbusy]

Can we use the extension from the extension Indexer to validate class? If possible, we can use it after the completion of this GSoC'24 project

Not sure what you mean. But we are talking about 2 different tool here (One plugin that can disable extension on a running controller and one tool that just display the available extension across all plugin)

[sridamul]

Not sure what you mean. But we are talking about 2 different tool here (One plugin that can disable extension on a running controller and one tool that just display the available extension across all plugin)

I thought that one can check whether the class names are available in the extension indexer. If it presents, then it is an extension and can be indicated with correct validation

As you suggested

https://github.com/jenkinsci/extension-filter-plugin/pull/70 By reading doc this can be achieved using an other class loader to check class existence

this is better approach.

[sridamul]

Can you take a look @sridamul

tested the code locally (both descriptor and extension) LGTM!

[sridamul]

If possible can you remove the default="&{null}"in the https://github.com/jenkinsci/extension-filter-plugin/blob/main/src/main/resources/org/jenkinsci/plugins/ConfigurableExtensionFilter/global.jelly#L12 as suggested by 'Daniel Beck'