Skip flaky test: test_validate_protected_queries__missing_fences

[devinfra-flakiness]

Please help us keep Sentry flake-free by merging this PR while a longer-term
resolution is implemented in issue #94922.

Test: tests/sentry/testutils/test_silo.py::test_validate_protected_queries__missing_fences

Statistics over the last 30 days:
* Runs Recorded: 760
* Flakes Failed: 0 (0%)
* Flakes Retried: 71 (9.342105%)

Example Flakes:

• https://github.com/getsentry/sentry/actions/runs/16075304134/job/45369004450
• https://github.com/getsentry/sentry/actions/runs/16013481824/job/45175716082
• https://github.com/getsentry/sentry/actions/runs/16010892038/job/45168269428
• https://github.com/getsentry/sentry/actions/runs/16009204224/job/45162909121
• https://github.com/getsentry/sentry/actions/runs/15981661613/job/45077217603

---

See also:
Flaky Test Policy & Responsibilities

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #94923       +/-   ##
===========================================
- Coverage   87.88%   77.12%   -10.76%     
===========================================
  Files       10449    10448        -1     
  Lines      604340   604311       -29     
  Branches    23547    23547               
===========================================
- Hits       531107   466077    -65030     
- Misses      72866   135852    +62986     
- Partials      367     2382     +2015