Make hardcoded value of HALF_MAX_FAILING_SIZE configurable

### What feature do you want to see added?

When the plug-in option `keepLongStdio` is set to `false` then output of failed tests is truncated to 100'000 characters. See hardcoded constant `HALF_MAX_FAILING_SIZE` in the class `CaseResult` https://github.com/jenkinsci/junit-plugin/blob/master/src/main/java/hudson/tasks/junit/CaseResult.java#L247.

It would be great to have it user-configurable. Many thanks in advance.

Currently we are forced to use `keepLongStdio: true` due to huge debug output of selected "end-to-end" JUnit tests. Output is important only in case failures so typically we waste resources.

### Upstream changes

_No response_