Chore: Add a shared test support layer (#14168)

All four Django apps have test code in common, but the only place to put it
was the documents app's own tests package, so paperless, paperless_ai and
paperless_mail each reached across an app boundary to import helpers and
relied on fixtures that were only defined for the documents test path.

This adds a root src/conftest.py holding the fixtures every app needs and an
new src/paperless_testing package for shared helpers a test names

src/conftest.py

@@ -0,0 +1,35 @@
+"""Fixtures available to every Paperless-ngx app.
+
+Loaded automatically for every test path. Keep module-scope imports minimal:
+this file is imported for every session,  so anything heavy belongs inside
+the fixture body that needs it.
+"""
+
+import pytest
+
+
+@pytest.fixture(scope="session", autouse=True)
+def faker_session_locale() -> str:
+    """Set Faker locale for reproducibility."""
+    return "en_US"
+
+
+@pytest.fixture(scope="session", autouse=True)
+def faker_seed() -> int:
+    return 12345
+
+
+@pytest.fixture(autouse=True)
+def _clear_content_type_caches() -> None:
+    """Clear Django's ContentType cache and guardian's lru_cache before each test.
+
+    Tests that delete and reinsert ContentType/Permission rows (e.g. the
+    importer) corrupt both caches. Without this fixture a subsequent test on
+    the same xdist worker sees stale ContentType objects and guardian raises
+    MixedContentTypeError.
+    """
+    from django.contrib.contenttypes.models import ContentType
+    from guardian.shortcuts import clear_ct_cache
+
+    ContentType.objects.clear_cache()
+    clear_ct_cache()