fix: don't default authSetting to ServerDefault in bulk user add

[jacalata]

Summary

• authSetting was hardcoded to "ServerDefault" when not explicitly set in the bulk user add request, which is incorrect for Tableau Cloud
• Changed to only send authSetting when explicitly set, matching the existing behaviour in single-user add/update

Fixes #1777

Test plan

• Verify bulk user add without explicit auth_setting no longer sends authSetting=ServerDefault in the request payload
• Verify bulk user add with explicit auth_setting still sends it correctly
• Verify Cloud bulk user add works without the erroneous default

🤖 Generated with Claude Code

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
tableauserverclient
__init__.py	5	0	100%
config.py	15	0	100%
datetime_helpers.py	25	1	1	96%
exponential_backoff.py	20	0	100%
filesys_helpers.py	31	0	100%
namespace.py	26	3	3	88%
tableauserverclient/bin
__init__.py	2	0	100%
_version.py	358	212	212	41%
tableauserverclient/helpers
__init__.py	1	0	100%
logging.py	2	0	100%
strings.py	31	1	1	97%
tableauserverclient/models
__init__.py	46	0	100%
collection_item.py	42	7	7	83%
column_item.py	55	32	32	42%
connection_credentials.py	35	11	11	69%
connection_item.py	86	13	13	85%
custom_view_item.py	144	21	21	85%
data_acceleration_report_item.py	54	1	1	98%
data_alert_item.py	159	4	4	97%
data_freshness_policy_item.py	156	15	15	90%
database_item.py	207	36	36	83%
datasource_item.py	301	12	12	96%
dqw_item.py	104	5	5	95%
exceptions.py	4	0	100%
extensions_item.py	132	4	4	97%
extract_item.py	45	4	4	91%
favorites_item.py	69	8	8	88%
fileupload_item.py	19	0	100%
flow_item.py	149	10	10	93%
flow_run_item.py	72	0	100%
group_item.py	89	6	6	93%
groupset_item.py	50	7	7	86%
interval_item.py	182	32	32	82%
job_item.py	188	10	10	95%
linked_tasks_item.py	80	1	1	99%
location_item.py	30	2	2	93%
metric_item.py	130	13	13	90%
oidc_item.py	64	3	3	95%
pagination_item.py	34	1	1	97%
permissions_item.py	112	12	12	89%
project_item.py	207	31	31	85%
property_decorators.py	100	18	18	82%
reference_item.py	26	2	2	92%
revision_item.py	60	1	1	98%
schedule_item.py	209	6	6	97%
server_info_item.py	37	4	4	89%
site_item.py	636	13	13	98%
subscription_item.py	101	2	2	98%
table_item.py	119	18	18	85%
tableau_auth.py	62	25	25	60%
tableau_types.py	27	1	1	96%
tag_item.py	15	0	100%
target.py	6	0	100%
task_item.py	57	2	2	96%
user_item.py	310	18	18	94%
view_item.py	220	16	16	93%
virtual_connection_item.py	64	8	8	88%
webhook_item.py	66	2	2	97%
workbook_item.py	362	16	16	96%
tableauserverclient/server
__init__.py	9	0	100%
exceptions.py	4	0	100%
filter.py	21	1	1	95%
pager.py	33	1	1	97%
query.py	143	18	18	87%
request_factory.py	1329	197	197	85%
request_options.py	385	5	5	99%
server.py	189	19	19	90%
sort.py	6	0	100%
tableauserverclient/server/endpoint
__init__.py	35	0	100%
auth_endpoint.py	75	10	10	87%
custom_views_endpoint.py	152	12	12	92%
data_acceleration_report_endpoint.py	21	0	100%
data_alert_endpoint.py	94	23	23	76%
databases_endpoint.py	111	30	30	73%
datasources_endpoint.py	328	34	34	90%
default_permissions_endpoint.py	44	3	3	93%
dqw_endpoint.py	45	16	16	64%
endpoint.py	187	18	18	90%
exceptions.py	70	6	6	91%
extensions_endpoint.py	31	0	100%
favorites_endpoint.py	95	22	22	77%
fileuploads_endpoint.py	51	0	100%
flow_runs_endpoint.py	62	9	9	85%
flow_task_endpoint.py	21	2	2	90%
flows_endpoint.py	210	54	54	74%
groups_endpoint.py	126	9	9	93%
groupsets_endpoint.py	72	7	7	90%
jobs_endpoint.py	68	9	9	87%
linked_tasks_endpoint.py	38	0	100%
metadata_endpoint.py	88	14	14	84%
metrics_endpoint.py	55	6	6	89%
oidc_endpoint.py	42	1	1	98%
permissions_endpoint.py	44	3	3	93%
projects_endpoint.py	157	24	24	85%
resource_tagger.py	127	36	36	72%
schedules_endpoint.py	119	11	11	91%
server_info_endpoint.py	36	10	10	72%
sites_endpoint.py	130	27	27	79%
subscriptions_endpoint.py	56	14	14	75%
tables_endpoint.py	110	36	36	67%
tasks_endpoint.py	63	6	6	90%
users_endpoint.py	184	9	9	95%
views_endpoint.py	150	9	9	94%
virtual_connections_endpoint.py	113	10	10	91%
webhooks_endpoint.py	54	9	9	83%
workbooks_endpoint.py	349	25	25	93%
TOTAL	11970	1425	88%