[Bug] Regression: cancelled conversation cannot be continued.

[ethanyhou]

Chat with Copilot and cancel, then continue chatting using the same session:

Error:

java.util.concurrent.CompletionException: org.eclipse.lsp4j.jsonrpc.ResponseErrorException: Request conversation/turn failed with message: Conversation with id 19873100-8901-41f7-a6e9-7eabf3edd451 does not exist
	at java.base/java.util.concurrent.CompletableFuture.encodeRelay(CompletableFuture.java:368)
	at java.base/java.util.concurrent.CompletableFuture.completeRelay(CompletableFuture.java:377)
	at java.base/java.util.concurrent.CompletableFuture$UniRelay.tryFire(CompletableFuture.java:1097)
	at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:510)
	at java.base/java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:2194)
	at org.eclipse.lsp4j.jsonrpc.RemoteEndpoint.handleResponse(RemoteEndpoint.java:220)
	at org.eclipse.lsp4j.jsonrpc.RemoteEndpoint.consume(RemoteEndpoint.java:204)
	at org.eclipse.lsp4e.LanguageServerWrapper.lambda$3(LanguageServerWrapper.java:419)
	at org.eclipse.lsp4j.jsonrpc.json.StreamMessageProducer.handleMessage(StreamMessageProducer.java:185)
	at org.eclipse.lsp4j.jsonrpc.json.StreamMessageProducer.listen(StreamMessageProducer.java:97)
	at org.eclipse.lsp4j.jsonrpc.json.ConcurrentMessageProcessor.run(ConcurrentMessageProcessor.java:114)
	at java.base/java.util.concurrent.Executors$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)
Caused by: org.eclipse.lsp4j.jsonrpc.ResponseErrorException: Request conversation/turn failed with message: Conversation with id 19873100-8901-41f7-a6e9-7eabf3edd451 does not exist
	... 11 more

[ethanyhou]

Seems like related to #166 , @xinyi-gong could you please take a second look?