fix(compat): get a v0.3 push config by task id alone (#1380)

## Problem

A v0.3 `pushNotificationConfig/get` with only a task id failed
validation, because the v1.0 request had no config id.

## Fix

When no config id is given, list the task's configs and return the one
stored under the task id, otherwise the first. This matches v0.3.

## Tests

`test_on_get_task_push_notification_config_by_task_id` runs through
`DefaultRequestHandlerV2` and `InMemoryPushNotificationConfigStore`. It
covers a set with and without an id and fails on main. 2329 passed.

Fixes #1379

---------

Co-authored-by: mykytanetipa <mykytanetipa@google.com>

Author: ConnorMoss02

src/a2a/compat/v0_3/request_handler.py

@@ -6,7 +6,7 @@
 from a2a.compat.v0_3 import types as types_v03
 from a2a.server.context import ServerCallContext
 from a2a.server.request_handlers.request_handler import RequestHandler
-from a2a.types.a2a_pb2 import Task
+from a2a.types.a2a_pb2 import ListTaskPushNotificationConfigsRequest, Task
 from a2a.utils import proto_utils as core_proto_utils
 from a2a.utils.errors import TaskNotFoundError
 
@@ -85,6 +85,24 @@ async def on_get_task_push_notification_config(
         v10_req = conversions.to_core_get_task_push_notification_config_request(
             request
         )
+        # v0.3 allows a get by task id alone. Prefer the config stored under
+        # the task id, which is where a config created without an id goes.
+        # Otherwise take the first, like v0.3 did.
+        if not v10_req.id:
+            v10_resp = await self.request_handler.on_list_task_push_notification_configs(
+                ListTaskPushNotificationConfigsRequest(task_id=v10_req.task_id),
+                context,
+            )
+            configs = list(v10_resp.configs)
+            v10_config = next(
+                (c for c in configs if c.id == v10_req.task_id),
+                configs[0] if configs else None,
+            )
+            if v10_config is None:
+                raise TaskNotFoundError
+            return conversions.to_compat_task_push_notification_config(
+                v10_config
+            )
         v10_config = (
             await self.request_handler.on_get_task_push_notification_config(
                 v10_req, context

tests/compat/v0_3/test_request_handler.py

@@ -2,10 +2,14 @@
 
 import pytest
 
+from a2a.auth.user import UnauthenticatedUser
 from a2a.compat.v0_3 import types as types_v03
 from a2a.compat.v0_3.request_handler import RequestHandler03
+from a2a.server.agent_execution import AgentExecutor
 from a2a.server.context import ServerCallContext
+from a2a.server.request_handlers import DefaultRequestHandlerV2
 from a2a.server.request_handlers.request_handler import RequestHandler
+from a2a.server.tasks import InMemoryPushNotificationConfigStore, TaskStore
 from a2a.types.a2a_pb2 import (
     AgentCapabilities,
     AgentCard,
@@ -241,6 +245,79 @@ async def test_on_get_task_push_notification_config(
     assert result.push_notification_config.url == 'http://example.com'
 
 
+@pytest.fixture
+def v03_handler_with_push_store():
+    task_store = AsyncMock(spec=TaskStore)
+    task_store.get.return_value = V10Task(id='task-1', context_id='ctx-1')
+    core_handler = DefaultRequestHandlerV2(
+        agent_executor=MagicMock(spec=AgentExecutor),
+        task_store=task_store,
+        push_config_store=InMemoryPushNotificationConfigStore(),
+        agent_card=AgentCard(
+            capabilities=AgentCapabilities(push_notifications=True)
+        ),
+    )
+    return RequestHandler03(request_handler=core_handler)
+
+
+@pytest.mark.anyio
+@pytest.mark.parametrize(
+    ('config_ids', 'expected_id'),
+    [
+        ([None], 'task-1'),
+        (['cfg-1'], 'cfg-1'),
+        (['cfg-1', None], 'task-1'),
+    ],
+)
+async def test_on_get_task_push_notification_config_by_task_id(
+    v03_handler_with_push_store, config_ids, expected_id
+):
+    """Without a config id, prefer the config stored under the task id."""
+    context = ServerCallContext(user=UnauthenticatedUser())
+    for config_id in config_ids:
+        await v03_handler_with_push_store.on_create_task_push_notification_config(
+            types_v03.SetTaskPushNotificationConfigRequest(
+                id='req-1',
+                method='tasks/pushNotificationConfig/set',
+                params=types_v03.TaskPushNotificationConfig(
+                    task_id='task-1',
+                    push_notification_config=types_v03.PushNotificationConfig(
+                        id=config_id, url='http://example.com'
+                    ),
+                ),
+            ),
+            context,
+        )
+
+    result = (
+        await v03_handler_with_push_store.on_get_task_push_notification_config(
+            types_v03.GetTaskPushNotificationConfigRequest(
+                id='req-2',
+                method='tasks/pushNotificationConfig/get',
+                params=types_v03.TaskIdParams(id='task-1'),
+            ),
+            context,
+        )
+    )
+
+    assert result.push_notification_config.id == expected_id
+
+
+@pytest.mark.anyio
+async def test_on_get_task_push_notification_config_by_task_id_none_set(
+    v03_handler_with_push_store,
+):
+    with pytest.raises(TaskNotFoundError):
+        await v03_handler_with_push_store.on_get_task_push_notification_config(
+            types_v03.GetTaskPushNotificationConfigRequest(
+                id='req-1',
+                method='tasks/pushNotificationConfig/get',
+                params=types_v03.TaskIdParams(id='task-1'),
+            ),
+            ServerCallContext(user=UnauthenticatedUser()),
+        )
+
+
 @pytest.mark.anyio
 async def test_on_create_task_push_notification_config(
     v03_handler, mock_core_handler, mock_context