fix(tests): Expect React login page in integration tests

[oioki]

Sentry now enables React authentication by default (getsentry/sentry#126486), which broke the integration tests on every branch, including master:

• / now ends at /auth/login/ instead of redirecting to /auth/login/sentry/.
• The React login page has no form to read csrfmiddlewaretoken from. The login fixture now takes the token from the sc CSRF cookie, which both login pages set.

The login POST is unchanged: only GET requests are served by the React page.

[github-actions]

Coverage Results 📊

✅ 22 passed | Total: 22 | Pass Rate: 100% | Execution Time: 9m 38s

📊 Comparison with Base Branch

Metric	Change
Total Tests	—
Passed Tests	—
Failed Tests	—
Skipped Tests	—

✨ Test counts unchanged from base.

All tests are passing successfully.

✅ Patch coverage is 100.00% (3 of 3 changed executable lines covered; target 50%).
Project statement coverage is 95.52% (down 0.01 percentage points from base (cd51aab) to head (43fdd1c)).

Changed files with executable lines (1)

File	Patch coverage	Changed executable lines
_integration-test/test_01_basics.py	100.00%	3/3 covered

Coverage diff

@@            Coverage Diff             @@
##        master     #4551       +/-##
==========================================
- Coverage    95.53%    95.52%    -0.01%
==========================================
  Files            5         5         —
  Tracked lines       336       335        -1
  Branches         0         0         —
==========================================
- Hits           321       320        -1
  Misses          15        15         —
  Partials         0         0         —

---

Generated by Coverage Action