No autocompletions in Java 1.8-based projects due to "Missing system library" error

This appears to be related to eclipse-jdt/eclipse.jdt.core#3298. I'm not sure when this changed but as of the update released yesterday I am now getting the following error in my logs any time I try to work on my Minecraft mod projects (e.g. glektarssza/minecraft-gtnh-customizer):

!ENTRY org.eclipse.jdt.core.manipulation 4 0 2025-10-29 09:43:37.638
!MESSAGE Error in JDT Core during AST creation
!STACK 0
java.lang.IllegalStateException: Missing system library
	at org.eclipse.jdt.core.dom.ASTParser.checkForSystemLibrary(ASTParser.java:311)
	at org.eclipse.jdt.core.dom.ASTParser.getClasspath(ASTParser.java:269)
	at org.eclipse.jdt.core.dom.ASTParser.internalCreateASTCached(ASTParser.java:1395)
	at org.eclipse.jdt.core.dom.ASTParser.lambda$1(ASTParser.java:1274)
	at org.eclipse.jdt.internal.core.JavaModelManager.cacheZipFiles(JavaModelManager.java:5709)
	at org.eclipse.jdt.core.dom.ASTParser.internalCreateAST(ASTParser.java:1274)
	at org.eclipse.jdt.core.dom.ASTParser.createAST(ASTParser.java:918)
	at org.eclipse.jdt.core.manipulation.CoreASTProvider$1.run(CoreASTProvider.java:294)
	at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:47)
	at org.eclipse.jdt.core.manipulation.CoreASTProvider.createAST(CoreASTProvider.java:286)
	at org.eclipse.jdt.core.manipulation.CoreASTProvider.getAST(CoreASTProvider.java:199)
	at org.eclipse.jdt.ls.core.internal.JDTUtils.getAst(JDTUtils.java:1953)
	at org.eclipse.jdt.ls.core.internal.handlers.SemanticTokensHandler.full(SemanticTokensHandler.java:45)
	at org.eclipse.jdt.ls.core.internal.handlers.JDTLanguageServer.lambda$57(JDTLanguageServer.java:1205)
	at org.eclipse.jdt.ls.core.internal.BaseJDTLanguageServer.lambda$0(BaseJDTLanguageServer.java:87)
	at java.base/java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:667)
	at java.base/java.util.concurrent.CompletableFuture$Completion.exec(CompletableFuture.java:504)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:511)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1450)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:2019)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:187)

To replicate the issue:

1. Ensure you have Git LFS installed properly (see this comment on redhat-developer/vscode-java#4218 for why).
2. Clone the repository.
3. Install the extensions outlined in the Enabled Extensions Information section below.
4. Copy the appropriate template VSCode settings from ./templates/vscode/ from inside the repository above into your ./.vscode/settings.json file inside the repository above.
5. Replace the Java paths inside the copied templates to a version of Java 21.
6. Ensure your JAVA_HOME environment variable is set to a version of Java 17.
7. Run ./gradlew setupDecompWorkspace inside the repository above.
8. Run ./gradlew build inside the repository above.
9. Run ./gradlew eclipse inside the repository above.
10. Try to get autocompletion in any file.
11. Observe the issue.

Start with the reproduction repository, its templates/vscode/ settings, and the Gradle commands listed in the issue. Trace the Missing system library failure through ASTParser, CoreASTProvider, JDTUtils, and SemanticTokensHandler, then compare behavior for the Java 1.8 project setup. Done means autocompletion works without the logged error after repeating the listed setup.