fix: add null check for list param in Files and SessionEvents

Files.listAsync and SessionEvents.listAsync called
param.toQueryString() without checking if param was null, causing
NPEs. All other listAsync methods already had this guard.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: foleydang

src/main/java/com/alibaba/dashscope/agentstudio/resource/Files.java

@@ -146,6 +146,9 @@ public CompletableFuture<AgentStudioFile> retrieveAsync(
   }
 
   public CompletableFuture<CursorPage<AgentStudioFile>> listAsync(FileListParam param) {
+    if (param == null) {
+      return AsyncHelper.failedFuture(new InputRequiredException("param is required!"));
+    }
     String query = param.toQueryString();
     String path = query.isEmpty() ? "files" : "files?" + query;
     GeneralServiceOption opt = AgentStudioConstants.newServiceOption(HttpMethod.GET, path, baseUrl);

src/main/java/com/alibaba/dashscope/agentstudio/resource/SessionEvents.java

@@ -98,6 +98,9 @@ public CompletableFuture<CursorPage<Message>> listAsync(
     if (sessionId == null || sessionId.isEmpty()) {
       return AsyncHelper.failedFuture(new InputRequiredException("sessionId is required!"));
     }
+    if (param == null) {
+      return AsyncHelper.failedFuture(new InputRequiredException("param is required!"));
+    }
     String query = param.toQueryString();
     String path = StringUtils.format("sessions/%s/events", sessionId);
     GeneralServiceOption opt =