Fix case sensitivity in GitHub inbound and outbound assignee sync

[jasonyuezhang]

Github usernames are case-insensitive and users currently can configure their usernames with any captialization they see fit.

To correctly attribute usernames to users, we need to do a case-insensitive search when looking for ExternalActor and send Github a case-insensitive username back as well. The second point is probably not needed but good to do.

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Copied from getsentry#101984
Original PR: getsentry#101984

[propel-test-bot]

Case-Insensitive GitHub Assignee Sync (Inbound & Outbound)

Makes user-mapping logic between Sentry and GitHub insensitive to username capitalization in both inbound and outbound assignment flows. Production code now lower-cases GitHub usernames before API calls and uses a case-insensitive lookup when resolving ExternalActor records. New tests cover inbound, outbound, and multi-group scenarios to verify the change.

Key Changes

• Updated sync_group_assignee_inbound_by_external_actor() in src/sentry/integrations/utils/sync.py to query ExternalActor.external_name with __iexact (case-insensitive)
• Enhanced GitHubIntegration.sync_assignee_outbound() in src/sentry/integrations/github/integration.py to lower() the stripped GitHub username before PATCHing the issue
• Added test test_assignment_with_external_actor_case_insensitive in tests/sentry/integrations/utils/test_sync.py
• Added test test_sync_assignee_outbound_case_insensitive in tests/sentry/integrations/github/test_integration.py

Affected Areas

• src/sentry/integrations/utils/sync.py
• src/sentry/integrations/github/integration.py
• Assignee sync test suites

This summary was automatically generated by @propel-code-bot