Fix memory leak from cancelled timeout tasks in CopilotSession

Replace Executors.newSingleThreadScheduledExecutor with an explicit
ScheduledThreadPoolExecutor so we can enable removeOnCancelPolicy(true).
Without this, each call to sendAndWait() that completes normally cancels
its timeout task, but the cancelled task remains in the scheduler's work
queue, leaking memory over the lifetime of the session.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Ed Burns <edburns@microsoft.com>

Author: edburns

src/main/java/com/github/copilot/sdk/CopilotSession.java

@@ -15,6 +15,7 @@
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.Executors;
 import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
 import java.util.concurrent.atomic.AtomicReference;
@@ -159,11 +160,13 @@ public final class CopilotSession implements AutoCloseable {
         this.sessionId = sessionId;
         this.rpc = rpc;
         this.workspacePath = workspacePath;
-        this.timeoutScheduler = Executors.newSingleThreadScheduledExecutor(r -> {
+        var executor = new ScheduledThreadPoolExecutor(1, r -> {
             var t = new Thread(r, "sendAndWait-timeout");
             t.setDaemon(true);
             return t;
         });
+        executor.setRemoveOnCancelPolicy(true);
+        this.timeoutScheduler = executor;
     }
 
     /**