Give sbt server enough heap for Scala Native tests

[mkurz]

Pull Request Checklist

• Read the contributor guidelines.
• Kept the change in one commit.
• Ran sbt validateCode and bash -n scripts/test-code.sh.
• No new files or API changes require copyright or documentation updates.

Purpose

Give the sbt server a 4 GB maximum heap for the testFull job by setting JAVA_OPTS. Preserve any existing JAVA_OPTS value and remove the ineffective -J RAM percentage options.

Background Context

The sbt 2 runner starts its native thin client by default. That client drops command-line -J options before starting the server, so the previous memory settings left the server at its default 1 GB heap. This caused out-of-memory failures during the Scala Native test job. JAVA_OPTS reaches the server at startup. A previous Java 21 / Scala 3.9 testFull run passed with the 4 GB setting.

Verification

• JAVA_OPTS=-Xmx4G sbt validateCode passed.
• bash -n scripts/test-code.sh and git diff --check passed.
• The test matrix runs scripts/test-code.sh on pull requests.

References

• Upstream sbt fix: [2.x] fix: Forward -J options to sbt server sbt/sbt#9839
• Affected pull requests: Test on Java 25 #1439 and Patch updates #1442