🐛 fix[gitlab]: sync assignees by external id

[iamrajjoshi]

Description

i am not fully certain this is the root cause of the GitLab assignment sync failures yet and haven't been able to repro

The leading hypothesis is that inbound assignment is sometimes trying to match by GitLab username when the stable mapped GitLab user ID is available, and outbound assignment can currently fail to resolve a GitLab assignee while still looking successful in lifecycle metrics.

This change updates GitLab assignee sync to prefer mapped GitLab user IDs, fall back to exact username matching, and halt outbound assignment sync when no valid GitLab assignee can be resolved.

Incase my hypothesis is wrong , i also added lifecycle extras around inbound and outbound assignment matching to debug more.

[iamrajjoshi]

temporary shim so i can try to get some more context

[iamrajjoshi]

temp should be thrown away

[github-actions]

Backend Test Failures

Failures on d50f0f5 in this run:

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

[gw1] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/models/test_groupassignee.py:286: 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 0x7f76cf166a50: id=9, organization_id=4558123004526624, integration_id=44, key='APP-123'>, None, assign=False, assignment_source=None)
E     Actual: sync_assignee_outbound(<ExternalIssue at 0x7f76daf9a4e0: id=9, organization_id=4558123004526624, integration_id=44, key='APP-123'>, None, assign=False, assignment_source=None, lifecycle=<sentry.integrations.utils.metrics.IntegrationEventLifecycle object at 0x7f76ee91d8d0>)

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

[gw0] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/models/test_groupassignee.py:165: 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 0x7fcb192aa0d0: id=18, organization_id=4558123004854288, integration_id=39, key='APP-123'>, RpcUser(id=98, pk=98, 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, 12, 4, 11, 56, 666949, 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=96, email='admin@localhost', is_verified=True)], authenticators=[]), assign=True, assignment_source=None)
E     Actual: sync_assignee_outbound(<ExternalIssue at 0x7fcb192a7070: id=18, organization_id=4558123004854288, integration_id=39, key='APP-123'>, RpcUser(id=98, pk=98, 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, 12, 4, 11, 56, 666949, 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=96, email='admin@localhost', is_verified=True)], authenticators=[]), assign=True, assignment_source=None, lifecycle=<sentry.integrations.utils.metrics.IntegrationEventLifecycle object at 0x7fcb194d5bd0>)