🐛 fix[gitlab]: add assignee sync diagnostics

[iamrajjoshi]

Description of the change

I am not fully certain we have the root cause of the GitLab assignment sync failures yet. Based on review and hand testing, this no longer changes assignee matching to prefer mapped external IDs or stricter username matching; it keeps the existing GitLab username search behavior.

This adds lifecycle context around inbound and outbound assignment sync so the next failure has enough detail to diagnose the mapping/search path. It preserves the existing no-op return behavior for GitLab outbound cases that previously returned early, including missing mapped external actors, invalid issue keys, GitLab user search failures, and empty GitLab user search results.

Test plan

• Python compile check for touched files
• Ruff check and format hooks for touched files
• Targeted mypy hook for touched source files
• Targeted pytest attempted locally, but Postgres/Redis devservices were not running in this worktree

Considerations

This is intentionally diagnostic: it preserves the current GitLab username resolution path while adding enough context to confirm or disprove the current assignment-sync hypothesis.

[GabeVillalobos]

This case will never execute. We don't set external ID for any of our GitLab mappings, so I'm not sure if this changes anything.

[GabeVillalobos]

I hand tested this API with my GitLab user, and this request should have returned a single user, so I can vouch that this is correct at least. Also, this API only returns a single username when a username is passed as query param, so the previous logic was slightly more correct: https://docs.gitlab.com/api/users/#as-a-regular-user

[github-actions]

Backend Test Failures

Failures on f844feb in this run:

tests/sentry/models/test_groupassignee.py::GroupAssigneeTestCase::test_assignee_sync_outbound_assign — log

[gw1] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/models/test_groupassignee.py:167: in test_assignee_sync_outbound_assign
    mock_sync_assignee_outbound.assert_called_with(
/opt/hostedtoolcache/Python/3.13.1/x64/lib/python3.13/unittest/mock.py:977: in assert_called_with
    raise AssertionError(_error_message()) from cause
E   AssertionError: expected call not found.
E   Expected: sync_assignee_outbound(<ExternalIssue at 0x7f5ef8b887d0: id=14, organization_id=4558229068513312, integration_id=38, key='APP-123'>, RpcUser(id=92, pk=92, name='', email='admin@localhost', username='admin@localhost', actor_id=None, display_name='admin@localhost', label='admin@localhost', is_superuser=True, is_authenticated=True, is_anonymous=False, is_active=True, is_staff=True, is_unclaimed=False, last_active=datetime.datetime(2026, 5, 30, 21, 45, 19, 878251, tzinfo=datetime.timezone.utc), is_sentry_app=False, password_usable=True, is_password_expired=False, is_suspended=False, roles=frozenset(), permissions=frozenset(), avatar=None, emails=frozenset({'admin@localhost'}), useremails=[RpcUserEmail(id=91, email='admin@localhost', is_verified=True)], authenticators=[]), assign=True, assignment_source=None)
E     Actual: sync_assignee_outbound(<ExternalIssue at 0x7f5ef92a8870: id=14, organization_id=4558229068513312, integration_id=38, key='APP-123'>, RpcUser(id=92, pk=92, name='', email='admin@localhost', username='admin@localhost', actor_id=None, display_name='admin@localhost', label='admin@localhost', is_superuser=True, is_authenticated=True, is_anonymous=False, is_active=True, is_staff=True, is_unclaimed=False, last_active=datetime.datetime(2026, 5, 30, 21, 45, 19, 878251, tzinfo=datetime.timezone.utc), is_sentry_app=False, password_usable=True, is_password_expired=False, is_suspended=False, roles=frozenset(), permissions=frozenset(), avatar=None, emails=frozenset({'admin@localhost'}), useremails=[RpcUserEmail(id=91, email='admin@localhost', is_verified=True)], authenticators=[]), assign=True, assignment_source=None, lifecycle=<sentry.integrations.utils.metrics.IntegrationEventLifecycle object at 0x7f5efbbf4520>)

tests/sentry/models/test_groupassignee.py::GroupAssigneeTestCase::test_assignee_sync_outbound_unassign — log

[gw0] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/models/test_groupassignee.py:288: in test_assignee_sync_outbound_unassign
    mock_sync_assignee_outbound.assert_called_with(
/opt/hostedtoolcache/Python/3.13.1/x64/lib/python3.13/unittest/mock.py:977: in assert_called_with
    raise AssertionError(_error_message()) from cause
E   AssertionError: expected call not found.
E   Expected: sync_assignee_outbound(<ExternalIssue at 0x7f99df528590: id=9, organization_id=4558229069692944, integration_id=45, key='APP-123'>, None, assign=False, assignment_source=None)
E     Actual: sync_assignee_outbound(<ExternalIssue at 0x7f99dcbf7e10: id=9, organization_id=4558229069692944, integration_id=45, key='APP-123'>, None, assign=False, assignment_source=None, lifecycle=<sentry.integrations.utils.metrics.IntegrationEventLifecycle object at 0x7f99df242900>)