fix(compat): return 400 for a non-integer v0.3 REST historyLength (#1382)

## Problem

A v0.3 REST `GetTask` with a non-integer `historyLength` returned 500,
because the `ValueError` from `int()` was not handled.

## Fix

Raise `InvalidParamsError` instead, with the same message the v1.0 REST
transport uses.

## Tests

`test_on_get_task_non_integer_history_length` and
`test_get_task_v03_non_integer_history_length` fail on main. 2325
passed.

Fixes #1381

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

Author: ConnorMoss02

src/a2a/compat/v0_3/rest_handler.py

@@ -28,6 +28,7 @@
 from a2a.compat.v0_3.request_handler import RequestHandler03
 from a2a.server.context import ServerCallContext
 from a2a.utils import constants
+from a2a.utils.errors import InvalidParamsError
 from a2a.utils.telemetry import SpanKind, trace_class
 from a2a.utils.version_validator import validate_version
 
@@ -250,7 +251,14 @@ async def on_get_task(
         """
         task_id = request.path_params['id']
         history_length_str = request.query_params.get('historyLength')
-        history_length = int(history_length_str) if history_length_str else None
+        try:
+            history_length = (
+                int(history_length_str) if history_length_str else None
+            )
+        except ValueError as e:
+            raise InvalidParamsError(
+                message=f'Failed to parse historyLength field: {e}.'
+            ) from e
 
         rpc_req = types_v03.GetTaskRequest(
             id='',

tests/compat/v0_3/test_rest_handler.py

@@ -9,6 +9,7 @@
 from a2a.server.context import ServerCallContext
 from a2a.server.request_handlers.request_handler import RequestHandler
 from a2a.types.a2a_pb2 import AgentCard
+from a2a.utils.errors import InvalidParamsError
 
 
 @pytest.fixture
@@ -315,6 +316,19 @@ async def test_on_get_task(rest_handler, mock_request, mock_context):
     assert called_req.params.history_length == 5
 
 
+@pytest.mark.anyio
+async def test_on_get_task_non_integer_history_length(
+    rest_handler, mock_request, mock_context
+):
+    mock_request.path_params = {'id': 'task-1'}
+    mock_request.query_params = {'historyLength': 'abc'}
+
+    with pytest.raises(InvalidParamsError):
+        await rest_handler.on_get_task(mock_request, mock_context)
+
+    rest_handler.handler03.on_get_task.assert_not_called()
+
+
 @pytest.mark.anyio
 async def test_list_push_notifications(
     rest_handler, mock_request, mock_context

tests/compat/v0_3/test_rest_routes_compat.py

@@ -199,6 +199,17 @@ async def test_get_task_v03(
     assert expected_response == actual_response
 
 
+@pytest.mark.anyio
+async def test_get_task_v03_non_integer_history_length(
+    client: AsyncClient, request_handler: MagicMock
+) -> None:
+    response = await client.get('/v1/tasks/test_task_id?historyLength=abc')
+
+    assert response.status_code == 400
+    assert response.json()['error']['status'] == 'INVALID_ARGUMENT'
+    request_handler.on_get_task.assert_not_called()
+
+
 @pytest.mark.anyio
 async def test_list_tasks_not_in_v03_adapter_routes(
     request_handler: RequestHandler,