Add unit test for release-specific SearchableEnvironment handling

[Copilot]

This PR adds a comprehensive unit test to validate the fix for issue eclipse-jdt#4399, which addresses how SearchableEnvironment determines whether to enable Java 9+ features (like module support) when processing release-specific source folders.

Background

In PR eclipse-jdt#4529, SearchableEnvironment.java was updated to check both the release parameter and the project compliance level when determining if Java 9+ features should be enabled:

boolean java9plus = release >= JavaProject.FIRST_MULTI_RELEASE || 
    JavaCore.callReadOnly(() -> CompilerOptions.versionToJdkLevel(project.getOption(JavaCore.COMPILER_COMPLIANCE, true)) >= ClassFileConstants.JDK9);

Previously, only the project compliance level was checked, which meant that source folders with release=9 or higher would not have Java 9+ features enabled if the overall project was set to Java 8 or lower.

Test Implementation

This PR adds testReleaseSpecificReconciliation() to ReconcilerMultiReleaseTests to verify the fix works correctly. The test:

1. Uses a Java 8 project with JCL18_LIB but includes source folders with release=9 and release=21 attributes
2. Tests cross-release type resolution by verifying that code in the release=21 folder can reference:
   • Types from the base source folder (release-agnostic)
   • Types and members from the release=9 folder
   • Types and members from the release=21 folder
3. Validates reconciliation succeeds without errors, confirming that SearchableEnvironment correctly uses the release attribute to enable Java 9+ features

Example Scenario

The test validates this multi-release structure:

// Base src/p/A.java - available at all levels
public class A { }

// src9/p/B.java - Java 9 version
public class B {
    A fieldA;  // Added in Java 9
}

// src9/p/C.java - Only available in Java 9+
class C { }

// src21/p/B.java - Java 21 version
public class B {
    A fieldA;
    C fieldC;  // Added in Java 21
    public void m() { }  // Added in Java 21
}

// src21/p/D.java - Test reconciliation
public class D {
    void m(B b) {
        A a = b.fieldA;  // OK: A from src, fieldA from src9+
        C c = b.fieldC;  // OK: C from src9, fieldC from src21+
        b.m();           // OK: m() from src21+
    }
}

The test ensures that reconciliation of D.java completes without errors, validating that the release-specific SearchableEnvironment correctly resolves all types and members across different release levels.

Fixes eclipse-jdt#4399

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• repo.eclipse.org
  • Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java --enable-native-access=ALL-UNNAMED -classpath /usr/share/apache-maven-3.9.11/boot/plexus-classworlds-2.9.0.jar -Dclassworlds.conf=/usr/share/apache-maven-3.9.11/bin/m2.conf -Dmaven.home=/usr/share/apache-maven-3.9.11 -Dlibrary.jansi.path=/usr/share/apache-maven-3.9.11/lib/jansi-native -Dmaven.multiModuleProjectDirectory=/home/REDACTED/work/eclipse.jdt.core/eclipse.jdt.core org.codehaus.plexus.classworlds.launcher.Launcher clean compile -pl org.eclipse.jdt.core.tests.model -am -DskipTests (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

Propose a unit test for the new feature

In eclipse-jdt#4529 there was asked for a new testcase in ReconcilerMultiReleaseTests, please suggest to enhance the test to cover the change in this branch.

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.