Fixes #35766

• getDifferentJdk() and getDifferentVersion() now throw IllegalStateException with actionable error messages instead of returning null
• Added lenient variants getDifferentJdkOrNull() and getDifferentVersionOrNull() for cases that need nullable returns
• Added isDifferentJdkAvailable() and isDifferentVersionAvailable() for use in test preconditions
• Updated IntegTestPreconditions to use the new boolean methods
• Added @Requires annotations to tests that rely on these methods

Context

See root issue for context.

Additional Notes

1. While working on this, I noticed several places where the precondition and actual usage seem mismatched (e.g., checking DifferentVersionAvailable but using getDifferentJdk()). I've flagged these with comments in the code. Could you clarify if these are bugs or intentional?

2. Does this approach match what was envisioned in the issue discussion?

3. Are the Messages of the thrown exceptions good enough?

4. Should I apply changes like this to other methods aswell ( for example 'getDifferentVersion(final Spec<? super JvmInstallationMetadata> filter)')?

5. All of the changes are to the tests themselves. I dont think I need to add unit tests is that correct ?

This is my first time contributing so any constructive feedback would be greatly appreciated.

Contributor Checklist

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• Provide integration tests (under <subproject>/src/integTest) to verify changes from a user perspective.
• Provide unit tests (under <subproject>/src/test) to verify logic.
• Update User Guide, DSL Reference, and Javadoc for public-facing changes.
• Ensure that tests pass sanity check: ./gradlew sanityCheck.
• Ensure that tests pass locally: ./gradlew <changed-subproject>:quickTest.

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