Describe the bug

The get-monthly-usage-attribution endpoint is returning a YYYY-MM formatted string under the usage[].month field.
This is the validation that fails as the month field is defined as datetime in the MonthlyUsageAttributionBody

Label the issue properly.

• Add severity/ - this is a breaking change on a stable public api, so considered quite severe.

To Reproduce

This snipped fails:

from datadog_api_client.v1.api.usage_metering_api import UsageMeteringApi
from datadog_api_client import ApiClient, Configuration

request = {
    "start_month": datetime(2025, 11, 4),
    "end_month": datetime(2025, 11, 5),
    "tag_breakdown_keys": "team",
    "fields": "*",
    "include_descendants": True
}

config = Configuration(
    host=host,
    api_key={"apiKeyAuth": api_key, "appKeyAuth": app_key},
)
with ApiClient(config) as client:
    api_instance = UsageMeteringApi(client)
    r = api_instance.get_monthly_usage_attribution(**request)

Expected behavior

for the sdk not to fail, and for the api to process correctly the response coming back from the dd-api.

Screenshots
If applicable, add screenshots to help explain your problem.

Environment and Versions (please complete the following information):
tested on 2.30.0 and 2.45.0. both failed with the same error

Additional context
This started failing at Nov 5'th and worked well up until this point.