test(vector): mark test_document_chunker as unit

Address claude-review round 3 on PR #868: add module-level
`pytestmark = pytest.mark.unit` so TestPageAwareChunker and
TestDocumentChunkerPositions are collected under `-m unit`, matching
test_processor_routing.py.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/unit/test_document_chunker.py

@@ -1,11 +1,15 @@
 """Unit tests for DocumentChunker with LangChain text splitters."""
 
+import pytest
+
 from nextcloud_mcp_server.vector.document_chunker import (
     ChunkWithPosition,
     DocumentChunker,
     PageAwareChunker,
 )
 
+pytestmark = pytest.mark.unit
+
 
 def _make_doc(pages: list[str]) -> tuple[str, list[dict]]:
     """Build (full_text, page_boundaries) the way the PDF extractors do.