fix: Cancel computation when Thread has been interrupted

[FlorianKroiss]

This is similar to #1486, but here our Thread is informed directly via org.eclipse.jface.text.contentassist.AsyncCompletionProposalPopup.cancelFutures(), which interrupts all running completion computations.

We spotted this rare problem within our application and noticed the following Error being logged:

ERROR 2026-01-28 08:50:18.708 [org.eclipse.jface.text-worker-1] org.eclipse.equinox.logger - InterruptedException
java.lang.InterruptedException: null
	at java.base/java.util.concurrent.CompletableFuture.reportGet(CompletableFuture.java:386)
	at java.base/java.util.concurrent.CompletableFuture.get(CompletableFuture.java:2073)
	at org.eclipse.lsp4e.operations.completion.LSContentAssistProcessor.computeCompletionProposals(LSContentAssistProcessor.java:177)
	at org.eclipse.jface.text.contentassist.AsyncCompletionProposalPopup.lambda$10(AsyncCompletionProposalPopup.java:385)
	at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:47)
	at org.eclipse.jface.text.contentassist.AsyncCompletionProposalPopup.lambda$9(AsyncCompletionProposalPopup.java:384)
	at org.eclipse.jface.text.contentassist.AsyncCompletionProposalPopup.lambda$11(AsyncCompletionProposalPopup.java:419)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:572)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:317)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
	at java.base/java.lang.Thread.run(Thread.java:1583)