fix: Fix mypy errors and enable streaming for hw example (#15)

Author: kthota-g

examples/helloworld/__main__.py

@@ -27,7 +27,7 @@
         version='1.0.0',
         defaultInputModes=['text'],
         defaultOutputModes=['text'],
-        capabilities=AgentCapabilities(),
+        capabilities=AgentCapabilities(streaming=True),
         skills=[skill],
         authentication=AgentAuthentication(schemes=['public']),
     )

src/a2a/server/agent_execution/context.py

@@ -19,7 +19,7 @@ def __init__(
         task_id: str | None = None,
         context_id: str | None = None,
         task: Task | None = None,
-        related_tasks: list[Task] = None,
+        related_tasks: list[Task] | None = None,
     ):
         if related_tasks is None:
             related_tasks = []

src/a2a/server/events/in_memory_queue_manager.py

@@ -18,7 +18,7 @@ class InMemoryQueueManager(QueueManager):
     true scalable deployment.
     """
 
-    def __init__(self):
+    def __init__(self) -> None:
         self._task_queue: dict[str, EventQueue] = {}
         self._lock = asyncio.Lock()