fix(tests): Stop test_global_config from leaking option overrides (#128459)

Fixes intermittent master failures of
`tests/relay_integration/test_integration.py::SentryRemoteTest::test_standalone_attachment`
(`assert None` in `post_and_retrieve_attachment`), e.g.
https://github.com/getsentry/sentry/actions/runs/39675570221.

**Cause**

`test_global_config` in
`tests/sentry/api/endpoints/test_relay_globalconfig_v3.py` uses the
`@override_options` decorator together with pytest-django's `settings`
fixture (added in #127378). The decorator's
`override_settings(SENTRY_OPTIONS=...)` exits before the fixture's
override is torn down. The fixture then restores the decorator's
settings holder, so the overridden `SENTRY_OPTIONS` stay in place for
every later test on that worker. After running it,
`options.get("relay.objectstore-attachments.sample-rate")` returns `1.0`
in an unrelated test instead of `0.0`, and the same goes for
`relay.attachment-inline.limit` and `relay.upload-max-chunk.size`. These
values reach the test Relay through the global config.

The leak has existed since Oct 15. It started failing on master after
#128359 added test files, which changed the shard plan. Part 0 of the
split `test_integration.py` moved from shard 14 to shard 10, which also
runs `test_relay_globalconfig_v3.py`. It fails only when xdist
`--dist=loadfile` puts both files on the same worker, which is why it
fails on some runs and not others. All 5 reruns fail because the leaked
state stays for the life of the worker.

**Fix**

Nest `override_settings` inside the test body so the two overrides
unwind in order. The test change is the only change.

**Verification** (local, `-n0`, Kafka/Snuba/Relay via devservices)

- `pytest test_relay_globalconfig_v3.py
test_integration.py::SentryRemoteTest::test_standalone_attachment`:
parent commit fails 1 of 10, fixed commit passes 10 of 10 (run twice)
- `test_standalone_attachment` alone passes on the parent commit
- `prek` is clean on the changed file

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via **dan**.
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Co-authored-by: sentry-junior[bot] <264270552+sentry-junior[bot]@users.noreply.github.com>
Co-authored-by: Dan Fuller <dfuller@sentry.io>

Author: sentry-junior[bot]

tests/sentry/api/endpoints/test_relay_globalconfig_v3.py

@@ -3,6 +3,7 @@
 
 import orjson
 import pytest
+from django.test import override_settings
 from django.urls import reverse
 from sentry_relay.processing import normalize_global_config
 
@@ -60,9 +61,11 @@ def inner(version, global_):
         "relay.upload-max-chunk.size": 123,
     }
 )
-def test_global_config(settings, inbound_filters_custom_error_values) -> None:
-    settings.SENTRY_INBOUND_FILTER_CUSTOM_VALUES = inbound_filters_custom_error_values
-    config = get_global_config()
+def test_global_config(inbound_filters_custom_error_values) -> None:
+    # Nest the settings override inside `override_options`: the `settings` fixture
+    # is torn down after the decorator exits, which leaks the options into later tests.
+    with override_settings(SENTRY_INBOUND_FILTER_CUSTOM_VALUES=inbound_filters_custom_error_values):
+        config = get_global_config()
 
     normalized = normalize_global_config(config)
     expected = deepcopy(config)