fix: use MySQL compatible ordering for listing tasks

Author: ishymko

src/a2a/server/tasks/database_task_store.py

@@ -189,8 +189,10 @@ async def list(
             count_stmt = select(func.count()).select_from(base_stmt.alias())
             total_count = (await session.execute(count_stmt)).scalar_one()
 
+            # Use coalesce to treat NULL timestamps as empty strings,
+            # which sort last in descending order
             stmt = base_stmt.order_by(
-                timestamp_col.desc().nulls_last(),
+                func.coalesce(timestamp_col, '').desc(),
                 self.task_model.id.desc(),
             )