Add form validation for max service and org name

There was a recent error in the logs because a service tried to change
its name to one exceeding 255 characters (which is a limit on the
database field). We can easily catch these errors on the form, so that
the user doesn't see an error page.

tests/app/main/views/test_add_service.py

@@ -288,30 +288,23 @@ def test_should_add_service_and_redirect_to_dashboard_when_existing_service(
     assert session['service_id'] == 101
 
 
-def test_should_return_form_errors_when_service_name_is_empty(
+@pytest.mark.parametrize('name, error_message', [
+    ('', 'Cannot be empty'),
+    ('.', 'Must include at least two alphanumeric characters'),
+    ('a' * 256, 'Service name must be 255 characters or fewer'),
+])
+def test_add_service_fails_if_service_name_fails_validation(
     client_request,
     mock_get_organisation_by_domain,
+    name,
+    error_message,
 ):
     page = client_request.post(
         'main.add_service',
-        _data={},
+        _data={"name": name},
         _expected_status=200,
     )
-    assert 'Cannot be empty' in page.text
-
-
-def test_add_service_fails_if_service_name_has_less_than_2_alphanumeric_characters(
-    client_request,
-    mock_get_organisation_by_domain,
-):
-    page = client_request.post(
-        'main.add_service',
-        _data={"name": "."},
-        _expected_status=200,
-    )
-
-    error_message = page.find("span", {"class": "govuk-error-message"}).text
-    assert 'Must include at least two alphanumeric characters' in error_message
+    assert error_message in page.find("span", {"class": "govuk-error-message"}).text
 
 
 def test_should_return_form_errors_with_duplicate_service_name_regardless_of_case(