When running tests on Java 26 disallow modification of final fields

[uschindler]

Description

JEP 500 is moving to making final fields really final. They are currently be allowed to be modified with deep reflection (using Field#setAccessible(true)) but as we do not allow setAccessible due to forbiddenapis, it is safe to further enforce this, especially to prevent dependencies from doing this. In JDK 26 the default is warn only and in later versions this will change to enforce.

For testing we should pass --illegal-final-field-mutation=deny to our test-runner staring with Java 26. We have similar stuff regarding preventing JNI in earlier Java versions, so the code to do this is already there.

I will check what needs to be done. I think we should wait a bit until the RC of JDK 26 comes out to not fail builds with older versions of JDK 26.

[uschindler]

The first Jenkins builds with JDK 26 ea+27 passed without final field modification warning, so I am confident that it passes with "deny", too!

[msokolov]

Although this was "allowed" it never really worked in any consistent way - you could modify a final field using reflection, but when I tried this I observed the modified value would not be visible in all threads.

[uschindler]

Thats why it if forbidden in Lucene's code from beginning, thanks to forbiddenAPis. This issue just looks into enabling the new feature to prevent it also in dependencies by making their tests fail.