Enable assertions to TaskList to catch potential bugs

Currently, the `TaskList` class methods lack assertions, which means potential bugs could go undetected during runtime, leading to unexpected behaviors and potential data corruption.

The lack of assertions makes the system vulnerable to subtle bugs that can be difficult to detect and diagnose.

Using assertions to catch potential bugs is a proactive approach to software development.

build.gradle

@@ -59,4 +59,5 @@ shadowJar {
 
 run {
     standardInput = System.in
+    enableAssertions = true
 }

src/main/java/alice/task/Task.java

@@ -66,6 +66,10 @@ public String getDescription() {
         return this.description;
     }
 
+    public boolean getIsDone() {
+        return this.isDone;
+    }
+
     /**
      * Marks the task as done.
      */

src/main/java/alice/task/TaskList.java

@@ -82,6 +82,7 @@ public List<Task> getTasks() {
      */
     public void add(Task task) {
         this.tasks.add(task);
+        assert this.tasks.contains(task) : "Task should be added to the list";
     }
 
     /**
@@ -99,7 +100,9 @@ public void delete(int index) {
      * @param index The index of the task to be marked as done.
      */
     public void markAsDone(int index) {
+
         this.tasks.get(index).markAsDone();
+        assert this.tasks.get(index).getIsDone() : "Task should be marked as done";
     }
 
     /**
@@ -109,6 +112,7 @@ public void markAsDone(int index) {
      */
     public void unmarkAsDone(int index) {
         this.tasks.get(index).unmarkAsDone();
+        assert !this.tasks.get(index).getIsDone() : "Task should be marked as not done";
     }
 
     /**