Chore: Move model factories to the shared test layer (#14170)

The model factories lived in the documents test package, but three other
apps needed them. The AI, mail and testing suites all reached across an app
boundary to import from documents.tests.factories, which made a private test
package into a shared dependency.

The factories now live in the shared testing package, where cross-app use is
the intended use.

src/paperless_ai/tests/test_chat.py

@@ -14,7 +14,6 @@ from llama_index.core.embeddings.mock_embed_model import MockEmbedding
 from llama_index.core.schema import TextNode
 
 from documents.models import Document
-from documents.tests.factories import DocumentFactory
 from paperless_ai import chat
 from paperless_ai import indexing
 from paperless_ai.chat import CHAT_ERROR_MESSAGE
@@ -22,6 +21,7 @@ from paperless_ai.chat import CHAT_METADATA_DELIMITER
 from paperless_ai.chat import _build_chat_prompt
 from paperless_ai.chat import _build_refine_prompt
 from paperless_ai.chat import stream_chat_with_documents
+from paperless_testing.factories import DocumentFactory
 
 if TYPE_CHECKING:
     from pathlib import Path