Add JDK cross-version compatibility tests

[pshevche]

Summary

Extends the Kotlin compatibility test suite to verify that the Spockk compiler plugin works correctly across different JDK versions.

Changes

• Extended Workspace fixture to support JDK version configuration via Gradle toolchain
• Updated SpockkKotlinCompatibilityTest to test combinations of:
  • 5 Kotlin versions: 1.8.22, 1.9.25, 2.0.21, 2.1.21, 2.2.21
  • 2 JDK versions: 21, 25
  • Total: 10 test combinations
• Configured CI to provision JDK 21 and 25 in the setup-gradle action
• Added JDK environment variable exports for test execution (JDK21, JDK25)

Test Coverage

The test suite verifies that the Spockk compiler plugin successfully transforms and executes specifications when compiling with different Kotlin compiler versions on different JDK runtimes.

Why JDK 21 and 25?

• JDK 21: Current LTS version, widely adopted
• JDK 25: Latest LTS version, ensures forward compatibility

Implementation Details

Workspace Fixture

Added optional jdkVersion parameter to setup() method that injects Gradle toolchain configuration:

java {
    toolchain {
        languageVersion = JavaLanguageVersion.of(jdkVersion)
    }
}

Test Structure

Used a data class to encapsulate version combinations:

data class KotlinAndJdkVersions(
    val kotlinVersion: String,
    val jdkVersion: Int
)

The test uses Spock's where block with a pre-computed list of all combinations for clean, maintainable test data.

CI Configuration

Moved JDK setup to the reusable setup-gradle action for consistency across workflows:

• Sets up JDK 21 (for Gradle daemon)
• Sets up JDK 25 (for cross-version testing)
• Exports environment variables for both JDKs

Test Plan

• Tests compile successfully
• Test structure verified (10 combinations generated)
• CI workflow provisions JDKs correctly
• Tests pass in CI with proper JDK environment variables

🤖 Generated with Claude Code

[pshevche]

Kotlin does not yet support the JDK25 target. So, this change will have to wait.