Handle duplicates in `dim_calendar_dates` (DbtTestFail: test.calitp_warehouse.unique_dim_calendar_dates__gtfs_key)

[cal-itp-sentry[bot]]

As a Cal-ITP GTFS data user, I want rows to be unique at the expected level so that they don't cause unexpected fanout in joins. Specifically, rows in dim_calendar_dates with duplicate gtfs_key values should be investigated and deduplicated

Sentry Issue: CAL-ITP-DATA-INFRA-26E8

DbtTestFail: test.calitp_warehouse.unique_dim_calendar_dates__gtfs_key.dfc084ed06 - Got 2 results, configured to fail if != 0