chore(hybrid-cloud): Mark extract_user_ids_from_mentions to be a region silo function

[dashed]

extract_user_ids_from_mentions() is only used in region silo endpoints. So I've updated the associated tests to be region silo stable.

[codecov]

Codecov Report

Merging #56441 (7cfc75b) into master (ff6e314) will decrease coverage by 0.02%.
Report is 33 commits behind head on master.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master   #56441      +/-   ##
==========================================
- Coverage   78.65%   78.63%   -0.02%     
==========================================
  Files        5076     5078       +2     
  Lines      218180   218363     +183     
  Branches    36929    36962      +33     
==========================================
+ Hits       171600   171719     +119     
- Misses      41038    41103      +65     
+ Partials     5542     5541       -1     

Files Changed	Coverage
.../sentry/api/endpoints/organization_events_trace.py	27.27%
.../sentry/api/serializers/rest_framework/mentions.py	100.00%