Change last token to '' and add more list_tasks tests

Author: lkawka

src/a2a/server/request_handlers/default_request_handler.py

@@ -144,7 +144,7 @@ async def on_list_tasks(
                 )
             processed_tasks.append(processed_task)
         return ListTasksResult(
-            next_page_token=page.next_page_token,
+            next_page_token=page.next_page_token or '',
             page_size=params.page_size or DEFAULT_LIST_TASKS_PAGE_SIZE,
             tasks=processed_tasks,
             total_size=page.total_size,

tests/server/tasks/test_database_task_store.py

@@ -190,12 +190,19 @@ async def test_get_task(db_store_parameterized: DatabaseTaskStore) -> None:
             5,
             '1',
         ),
-        # Pagination (second page)
+        # Pagination (final page)
         (
-            ListTasksParams(page_size=2, page_token='1'),
-            ['task-2', 'task-1'],
+            ListTasksParams(page_size=2, page_token='2'),
+            ['task-0'],
             5,
-            '2',
+            None,
+        ),
+        # Pagination (out of bounds)
+        (
+            ListTasksParams(page_size=2, page_token='3'),
+            [],
+            5,
+            None,
         ),
         # Filtering by context_id
         (

tests/server/tasks/test_inmemory_task_store.py

@@ -50,12 +50,19 @@ async def test_in_memory_task_store_get_nonexistent() -> None:
             5,
             '1',
         ),
-        # Pagination (second page)
+        # Pagination (final page)
         (
-            ListTasksParams(page_size=2, page_token='1'),
-            ['task-2', 'task-3'],
+            ListTasksParams(page_size=2, page_token='2'),
+            ['task-4'],
             5,
-            '2',
+            None,
+        ),
+        # Pagination (out of bounds)
+        (
+            ListTasksParams(page_size=2, page_token='3'),
+            [],
+            5,
+            None,
         ),
         # Filtering by context_id
         (