Small string fixes for encoding compatibility issues

[headius]

See ruby/rake#619. The first two fixes here are in service of fixing that failure, but are not the actual bug fix. See jruby/jruby#8711 for the actual fix.

The other two fixes get jruby-head running green in CI.

• Don't re-raise catString exceptions as a Java RuntimeException. This caused the ugly output in Bump ruby/setup-ruby from 1.222.0 to 1.227.0 rake#619.
• Re-get the encoding when the string to be written in write must be converted first, since it may be negotiated to something other than its original encoding.
• Add setup-java to jruby-head targets to ensure Java 21 is in PATH for JRuby 10.
• Fix modifiable check for null StringIO (from eb4ee49).

[headius]

These fixes should be released ASAP once green. The bad null StringIO code will not segfault on JRuby, but it will produce an internal Java NullPointerException when checking modifiability (which we consider equivalent to a "crash").

[headius]

All set!

@hsbt@nobu@kou Hello! I would like a release if possible, or I can do a release if you permit me.

[kou]

I can do but how about you try it?
Here is the current release procedure:

1. Add a release note to NEWS.md something like f8fcaa8
2. Push it
3. Run rake release (gems are published from GitHub Actions)

[headius]

@kou I will give it a try and let you know when it's done.

[headius]

@kou Apparently I do not have push rights for the gem and rake release does not build and push the -java version. Help?

[headius]

Actually it looks like it tries to release but fails during the build: https://github.com/ruby/stringio/actions/runs/14066275506/job/39389435498#step:3:278

I'm not sure what this error is so I'll have to investigate separately. We need to get the -java gem out there now though.

[kou]

No problem. I'll fix it and release a new version.

[kou]

It worked when I re-run the job: https://github.com/ruby/stringio/actions/runs/14066275506/job/39408925962

[kou]

BTW, let's run the CI jobs with pull request to detect problems before we release a new version: #127

[headius]

BTW, let's run the CI jobs with pull request to detect problems before we release a new version: #127

Great idea!

Thanks for your help!