typing: graduate tests.test_admin* chunk out of the mypy baseline (#1738)

[feiiiiii5]

Part of #1738. Graduates the tests.test_admin* chunk (2 files) out of the mypy baseline — the Auth0 admin authentication tests + the Django admin views tests.

What changed

• mypy.ini — removed 2 ignore_errors blocks:
  • [mypy-opencontractserver.tests.test_admin]
  • [mypy-opencontractserver.tests.test_admin_auth]
• docs/typing/mypy_baseline.txt — pruned the corresponding 122 lines (2942 → 2820).

Per-file fixes

test_admin.py (6 errors)

• Swapped the module-level User = get_user_model() alias for the concrete from opencontractserver.users.models import User import (mypy rejects a get_user_model() variable as a type annotation).
• Widened two helper signatures from obj: object to obj: Model so _meta / pk resolve.
• # type: ignore[arg-type] on CorpusAdmin(Corpus, None) — ModelAdmin's second-arg AdminSite is genuinely optional at runtime but typed as required.
• # type: ignore[method-assign] on the Mock() reassignment of message_user — intentional test isolation.

test_admin_auth.py (116 errors)

• Same get_user_model() → direct import swap.
• Class-level annotations on 7 TestCase classes that store User instances on setUpTestData(cls):
  • TestAdminClaimsSync.user
  • TestAuth0SuperuserAllowlist.user
  • TestAuth0AdminBackend.{staff_user, non_staff_user, inactive_staff, superuser}
  • TestAdminLoginView.{staff_user, regular_user}
  • TestAdminLogoutView.staff_user
  • TestGetUserByPayloadWithClaimSync.user
  • TestAdminLoginRateLimit.staff_user
• 17 # type: ignore[attr-defined] on response.url redirects — django-stubs' _MonkeyPatchedWSGIResponse doesn't expose .url even though Django's test client always sets it on 302s.

Verification

• mypy --config-file mypy.ini opencontractserver config → `Success: no issues found in 1513 source files` (both pre-commit mypy==2.0.0 and CI mypy==2.1.0 pins).
• python scripts/collate_changelog.py --check → `OK: 252 changelog fragment(s) valid`.
• No production code changed.

Patterns used (consistent with earlier #1738 graduation passes)

• get_user_model() → direct import: established in the conversation & threading pass.
• Class-level setUpTestData annotations: recommended fix from typing: graduate opencontractserver.discovery.tests.test_discovery_views from mypy baseline #1479.
• # type: ignore[attr-defined] on _MonkeyPatchedWSGIResponse.url: stub gap (test client always sets .url on 302s).

[feiiiiii5]

Closing this typing cleanup — I'm trimming my open PRs and won't be actively following up on it. The mypy-baseline graduation stands on its own if it's wanted later; the branch stays on my fork for an easy reopen. Thanks!