test(django): Remove @pytest.mark.forked dependency from Django integration tests

Bắt đầu với fixture sentry_init và DjangoIntegration.setup_once, sau đó kiểm tra tests/integrations/django/test_basic.py, test_cache_module.py, test_db_query_data.py, test_db_transactions.py và test_data_scrubbing.py. Xóa các marker forked từng file một, thêm cơ chế cô lập tường minh cho Django và trạng thái tích hợp, rồi chạy các test bị ảnh hưởng sau mỗi lần migration. Công việc được hoàn tất khi tất cả test được liệt kê đều pass mà không cần pytest-forked và không có rò rỉ trạng thái giữa các test.

Background

The Django integration test suite uses @pytest.mark.forked (via pytest-forked) on 52 tests across 5 files to isolate global state. This dates back to #522 ("Less boxed testing") and was a pragmatic substitute for proper teardown.

Problem

pytest-forked is actively harmful in some scenarios:

• Python 3.12 + asyncio + GitHub Actions: Forking a multi-threaded process and then running an asyncio loop in the child causes silent hangs (the loop inherits dead threads). This consumed the full 30-minute CI budget on the Django ASGI tests until #6410 dropped forked from those 10 tests.
• Slow: Fork-per-test multiplies suite runtime.
• Hides real isolation bugs: Tests pass under fork but would fail under proper sequential execution, masking integration setup that isn't idempotent.

Goal

Remove @pytest.mark.forked from every test under tests/integrations/django/ and replace it with explicit isolation.

Proposed approach

1. Extend (or autouse-wrap) the sentry_init fixture so the non-forked path also resets:
   • sentry_sdk.integrations._processed_integrations
   • sentry_sdk.integrations._installed_integrations
2. Add a reset_django_state fixture that snapshots and restores:
   • settings.MIDDLEWARE
   • Django signal receivers (request_started, request_finished, etc.)
   • Anything else DjangoIntegration.setup_once connects
3. Audit DjangoIntegration.setup_once (and submodule patches under sentry_sdk/integrations/django/) for non-idempotent module-level patching.
4. Migrate test files one at a time: drop the marker, run, fix the resulting leaks, repeat.

Current state

Files still using @pytest.mark.forked under tests/integrations/django/ (count via grep -rc):

• test_basic.py (13)
• test_cache_module.py (14)
• test_db_query_data.py (10)
• test_db_transactions.py (12)
• test_data_scrubbing.py (3)

The ASGI subset (tests/integrations/django/asgi/test_asgi.py) was removed in #6410.