fix: return entire history when history_length=0 (#537)

# Description

The specification states that a history length of 0 should return
unlimited results (see
[code](https://github.com/a2aproject/A2A/blob/202aa069e66f701bacf2156d42d8916fc96a5188/specification/grpc/a2a.proto#L128-L130)).
However, this was recently changed to return 0 results. This fix
restores the correct behavior.

Please note that there is an outstanding proposal to change this
behavior. See a2aproject/A2A#1071 for more
details.

Prerequisites:
- [x] Follow the [`CONTRIBUTING`
Guide](https://github.com/a2aproject/a2a-python/blob/main/CONTRIBUTING.md).
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<https://www.conventionalcommits.org/> specification.
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- `feat!:`, or `fix!:`, `refactor!:`, etc., which represent a breaking
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- [x] Ensure the tests and linter pass (Run `bash scripts/format.sh`
from the repository root to format)
- [x] Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

Author: lkawka

src/a2a/utils/task.py

@@ -83,11 +83,9 @@ def apply_history_length(task: Task, history_length: int | None) -> Task:
         A new task object with limited history
     """
     # Apply historyLength parameter if specified
-    if history_length is not None and task.history:
+    if history_length is not None and history_length > 0 and task.history:
         # Limit history to the most recent N messages
-        limited_history = (
-            task.history[-history_length:] if history_length > 0 else []
-        )
+        limited_history = task.history[-history_length:]
         # Create a new task instance with limited history
         return task.model_copy(update={'history': limited_history})
 

tests/server/request_handlers/test_default_request_handler.py

@@ -834,6 +834,11 @@ async def test_on_message_send_non_blocking():
 
     assert task is not None
     assert task.status.state == TaskState.completed
+    assert (
+        result.history
+        and task.history
+        and len(result.history) == len(task.history)
+    )
 
 
 @pytest.mark.asyncio
@@ -855,7 +860,7 @@ async def test_on_message_send_limit_history():
         configuration=MessageSendConfiguration(
             blocking=True,
             accepted_output_modes=['text/plain'],
-            history_length=0,
+            history_length=1,
         ),
     )
 
@@ -866,17 +871,17 @@ async def test_on_message_send_limit_history():
     # verify that history_length is honored
     assert result is not None
     assert isinstance(result, Task)
-    assert result.history is not None and len(result.history) == 0
+    assert result.history is not None and len(result.history) == 1
     assert result.status.state == TaskState.completed
 
     # verify that history is still persisted to the store
     task = await task_store.get(result.id)
     assert task is not None
-    assert task.history is not None and len(task.history) > 0
+    assert task.history is not None and len(task.history) > 1
 
 
 @pytest.mark.asyncio
-async def test_on_task_get_limit_history():
+async def test_on_get_task_limit_history():
     task_store = InMemoryTaskStore()
     push_store = InMemoryPushNotificationConfigStore()
 
@@ -892,7 +897,8 @@ async def test_on_task_get_limit_history():
             parts=[Part(root=TextPart(text='Hi'))],
         ),
         configuration=MessageSendConfiguration(
-            blocking=True, accepted_output_modes=['text/plain']
+            blocking=True,
+            accepted_output_modes=['text/plain'],
         ),
     )
 
@@ -904,14 +910,14 @@ async def test_on_task_get_limit_history():
     assert isinstance(result, Task)
 
     get_task_result = await request_handler.on_get_task(
-        TaskQueryParams(id=result.id, history_length=0),
+        TaskQueryParams(id=result.id, history_length=1),
         create_server_call_context(),
     )
     assert get_task_result is not None
     assert isinstance(get_task_result, Task)
     assert (
         get_task_result.history is not None
-        and len(get_task_result.history) == 0
+        and len(get_task_result.history) == 1
     )