Add closed-session guard implementation

Co-authored-by: brunoborges <129743+brunoborges@users.noreply.github.com>

Author: Copilot

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

@@ -97,6 +97,9 @@ public final class CopilotSession implements AutoCloseable {
     private final AtomicReference<SessionHooks> hooksHandler = new AtomicReference<>();
     private volatile EventErrorHandler eventErrorHandler;
     private volatile EventErrorPolicy eventErrorPolicy = EventErrorPolicy.PROPAGATE_AND_LOG_ERRORS;
+    
+    /** Tracks whether this session instance has been terminated via close(). */
+    private volatile boolean isTerminated = false;
 
     /**
      * Creates a new session with the given ID and RPC client.
@@ -186,11 +189,13 @@ public String getWorkspacePath() {
      * @param handler
      *            the error handler, or {@code null} to use only the default logging
      *            behavior
+     * @throws IllegalStateException if this session has been terminated
      * @see EventErrorHandler
      * @see #setEventErrorPolicy(EventErrorPolicy)
      * @since 1.0.8
      */
     public void setEventErrorHandler(EventErrorHandler handler) {
+        ensureNotTerminated();
         this.eventErrorHandler = handler;
     }
 
@@ -224,11 +229,13 @@ public void setEventErrorHandler(EventErrorHandler handler) {
      * @param policy
      *            the error policy (default is
      *            {@link EventErrorPolicy#PROPAGATE_AND_LOG_ERRORS})
+     * @throws IllegalStateException if this session has been terminated
      * @see EventErrorPolicy
      * @see #setEventErrorHandler(EventErrorHandler)
      * @since 1.0.8
      */
     public void setEventErrorPolicy(EventErrorPolicy policy) {
+        ensureNotTerminated();
         if (policy == null) {
             throw new NullPointerException("policy must not be null");
         }
@@ -244,9 +251,11 @@ public void setEventErrorPolicy(EventErrorPolicy policy) {
      * @param prompt
      *            the message text to send
      * @return a future that resolves with the message ID assigned by the server
+     * @throws IllegalStateException if this session has been terminated
      * @see #send(MessageOptions)
      */
     public CompletableFuture<String> send(String prompt) {
+        ensureNotTerminated();
         return send(new MessageOptions().setPrompt(prompt));
     }
 
@@ -260,9 +269,11 @@ public CompletableFuture<String> send(String prompt) {
      *            the message text to send
      * @return a future that resolves with the final assistant message event, or
      *         {@code null} if no assistant message was received
+     * @throws IllegalStateException if this session has been terminated
      * @see #sendAndWait(MessageOptions)
      */
     public CompletableFuture<AssistantMessageEvent> sendAndWait(String prompt) {
+        ensureNotTerminated();
         return sendAndWait(new MessageOptions().setPrompt(prompt));
     }
 
@@ -275,10 +286,12 @@ public CompletableFuture<AssistantMessageEvent> sendAndWait(String prompt) {
      * @param options
      *            the message options containing the prompt and attachments
      * @return a future that resolves with the message ID assigned by the server
+     * @throws IllegalStateException if this session has been terminated
      * @see #sendAndWait(MessageOptions)
      * @see #send(String)
      */
     public CompletableFuture<String> send(MessageOptions options) {
+        ensureNotTerminated();
         var request = new SendMessageRequest();
         request.setSessionId(sessionId);
         request.setPrompt(options.getPrompt());
@@ -304,10 +317,12 @@ public CompletableFuture<String> send(MessageOptions options) {
      *         {@code null} if no assistant message was received. The future
      *         completes exceptionally with a TimeoutException if the timeout
      *         expires.
+     * @throws IllegalStateException if this session has been terminated
      * @see #sendAndWait(MessageOptions)
      * @see #send(MessageOptions)
      */
     public CompletableFuture<AssistantMessageEvent> sendAndWait(MessageOptions options, long timeoutMs) {
+        ensureNotTerminated();
         var future = new CompletableFuture<AssistantMessageEvent>();
         var lastAssistantMessage = new AtomicReference<AssistantMessageEvent>();
 
@@ -365,9 +380,11 @@ public CompletableFuture<AssistantMessageEvent> sendAndWait(MessageOptions optio
      *            the message options containing the prompt and attachments
      * @return a future that resolves with the final assistant message event, or
      *         {@code null} if no assistant message was received
+     * @throws IllegalStateException if this session has been terminated
      * @see #sendAndWait(MessageOptions, long)
      */
     public CompletableFuture<AssistantMessageEvent> sendAndWait(MessageOptions options) {
+        ensureNotTerminated();
         return sendAndWait(options, 60000);
     }
 
@@ -397,11 +414,13 @@ public CompletableFuture<AssistantMessageEvent> sendAndWait(MessageOptions optio
      * @param handler
      *            a callback to be invoked when a session event occurs
      * @return a Closeable that, when closed, unsubscribes the handler
+     * @throws IllegalStateException if this session has been terminated
      * @see #on(Class, Consumer)
      * @see AbstractSessionEvent
      * @see #setEventErrorPolicy(EventErrorPolicy)
      */
     public Closeable on(Consumer<AbstractSessionEvent> handler) {
+        ensureNotTerminated();
         eventHandlers.add(handler);
         return () -> eventHandlers.remove(handler);
     }
@@ -447,10 +466,12 @@ public Closeable on(Consumer<AbstractSessionEvent> handler) {
      * @param handler
      *            a callback invoked when events of this type occur
      * @return a Closeable that unsubscribes the handler when closed
+     * @throws IllegalStateException if this session has been terminated
      * @see #on(Consumer)
      * @see AbstractSessionEvent
      */
     public <T extends AbstractSessionEvent> Closeable on(Class<T> eventType, Consumer<T> handler) {
+        ensureNotTerminated();
         Consumer<AbstractSessionEvent> wrapper = event -> {
             if (eventType.isInstance(event)) {
                 handler.accept(eventType.cast(event));
@@ -708,9 +729,11 @@ CompletableFuture<Object> handleHooksInvoke(String hookType, JsonNode input) {
      * assistant responses, tool invocations, and other session events.
      *
      * @return a future that resolves with a list of all session events
+     * @throws IllegalStateException if this session has been terminated
      * @see AbstractSessionEvent
      */
     public CompletableFuture<List<AbstractSessionEvent>> getMessages() {
+        ensureNotTerminated();
         return rpc.invoke("session.getMessages", Map.of("sessionId", sessionId), GetMessagesResponse.class)
                 .thenApply(response -> {
                     var events = new ArrayList<AbstractSessionEvent>();
@@ -737,20 +760,40 @@ public CompletableFuture<List<AbstractSessionEvent>> getMessages() {
      * continuing to generate a response.
      *
      * @return a future that completes when the abort is acknowledged
+     * @throws IllegalStateException if this session has been terminated
      */
     public CompletableFuture<Void> abort() {
+        ensureNotTerminated();
         return rpc.invoke("session.abort", Map.of("sessionId", sessionId), Void.class);
     }
+    
+    /**
+     * Verifies that this session has not yet been terminated.
+     * 
+     * @throws IllegalStateException if close() has already been invoked
+     */
+    private void ensureNotTerminated() {
+        if (isTerminated) {
+            throw new IllegalStateException("Session is closed");
+        }
+    }
 
     /**
      * Disposes the session and releases all associated resources.
      * <p>
      * This destroys the session on the server, clears all event handlers, and
      * releases tool and permission handlers. After calling this method, the session
-     * cannot be used again.
+     * cannot be used again. Subsequent calls to this method have no effect.
      */
     @Override
     public void close() {
+        synchronized (this) {
+            if (isTerminated) {
+                return; // Already terminated - no-op
+            }
+            isTerminated = true;
+        }
+
         try {
             rpc.invoke("session.destroy", Map.of("sessionId", sessionId), Void.class).get(5, TimeUnit.SECONDS);
         } catch (Exception e) {