Fix test on user_id_or_client_key in fct_ga4__user_ids model

[adamribaudo-velir]

Description & motivation

Closes #191

@TannerHopkins noticed that the uniqueness test for fct_ga4__user_ids can fail when joining together multiple data streams as it checks only the user_id_or_client_key. The client_key value already takes the stream_id values into account, but user_id does not. This means that sharing a user_id across streams would invalidate the test (despite being a valid outcome).

The test has been updated to incorporate stream_id in the uniqueness check.

Checklist

• [x] I have verified that these changes work locally
• [na] I have updated the README.md (if applicable)
• [x] I have added tests & descriptions to my models (and macros if applicable)
• [x] I have run dbt test and python -m pytest . to validate existing tests

[adamribaudo-velir]

@TannerHopkins this should be all set if you want to retest