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.

Bắt đầu với repository tái hiện lỗi, các thiết lập trong templates/vscode/ và những lệnh Gradle được liệt kê trong issue. Theo dõi lỗi Missing system library qua ASTParser, CoreASTProvider, JDTUtils và SemanticTokensHandler, sau đó so sánh hành vi với thiết lập dự án Java 1.8. Công việc được xem là hoàn tất khi tính năng tự động hoàn thành hoạt động mà không có lỗi được ghi lại sau khi lặp lại phần thiết lập đã liệt kê.