test(chunking): add doc_items field to ChunkResult serialization test

The `doc_items` field was added to `ChunkResult` earlier in the
feature branch (used by ChunkWriter for DERIVED_FROM edges), but the
test fixture was never updated. CI caught it now that the PR is open.

Fixes: tests/test_chunking.py::TestChunkResult::test_serializable

document-parser/tests/test_chunking.py

@@ -66,6 +66,7 @@ class TestChunkResult:
             "source_page": 1,
             "token_count": 10,
             "bboxes": [],
+            "doc_items": [],
         }