feat: send task as a first subscribe event

Following

> The operation MUST return a Task object as the first event in the stream

introduced in 1.0: https://a2a-protocol.org/latest/specification/#316-subscribe-to-task

Author: ishymko

src/a2a/server/request_handlers/default_request_handler.py

@@ -555,11 +555,15 @@ async def on_subscribe_to_task(
 
         if task.status.state in TERMINAL_TASK_STATES:
             raise ServerError(
-                error=InvalidParamsError(
+                error=UnsupportedOperationError(
                     message=f'Task {task.id} is in terminal state: {task.status.state}'
                 )
             )
 
+        # The operation MUST return a Task object as the first event in the stream
+        # https://a2a-protocol.org/latest/specification/#316-subscribe-to-task
+        yield task
+
         task_manager = TaskManager(
             task_id=task.id,
             context_id=task.context_id,

tests/server/request_handlers/test_default_request_handler.py

@@ -1499,6 +1499,9 @@ async def exec_side_effect(_request, queue: EventQueue):
     # Allow producer to emit the next event
     allow_second_event.set()
 
+    first_subscribe_event = await anext(resub_gen)
+    assert first_subscribe_event == task_for_resub
+
     received = await resub_gen.__anext__()
     assert received == second_event
 
@@ -2706,7 +2709,7 @@ async def test_on_subscribe_to_task_in_terminal_state(terminal_state):
         async for _ in request_handler.on_subscribe_to_task(params, context):
             pass  # pragma: no cover
 
-    assert isinstance(exc_info.value.error, InvalidParamsError)
+    assert isinstance(exc_info.value.error, UnsupportedOperationError)
     assert exc_info.value.error.message
     assert (
         f'Task {task_id} is in terminal state: {terminal_state}'