feat: re-export SettingsWrapper from pytest_django for typing (#1257)

[SAY-5]

Closes #1257.

SettingsWrapper is now importable from the top-level pytest_django namespace so the settings fixture can be type-annotated without reaching into pytest_django.fixtures:

from pytest_django import SettingsWrapper

def test_with_specific_settings(settings: SettingsWrapper):
    ...

Mirrors the existing pattern used for DjangoAssertNumQueries and DjangoCaptureOnCommitCallbacks.

• adds SettingsWrapper to pytest_django.__all__
• documents the typing usage under the settings fixture in docs/helpers.rst
• changelog entry under v4.12.1
• regression test in TestSettings