fix(mem_reader): pass sources=[] explicitly when creating split chunk items

Dropping the sources kwarg entirely broke the _make_memory_item call-site
contract: callers intercepting the keyword with a strict signature (e.g.
the existing project-id propagation test whose fake requires sources=)
fail, and the broad except in _split_large_memory_item turns the TypeError
into a silent fall-back to the unsplit item, so large items stopped being
chunked at all.

Passing an explicit empty list keeps the #2453 no-duplication behaviour
identical while restoring the stable kwarg contract.

Also drop the vacuous no-parent-sources test case: with an empty parent
sources list the old and new code both produced sources == [], so the
assertion could never distinguish the bug from the fix. The non-vacuous
test_split_chunks_do_not_carry_parent_sources already pins the invariant
with a parent that does carry sources.

Author: RerankerGuo

src/memos/mem_reader/multi_modal_struct.py

@@ -160,8 +160,8 @@ def _create_chunk_item(chunk_idx: int, chunk):
                     "chunk_total": chunk_total,
                 }
                 # Create a new memory item for each chunk, preserving original
-                # metadata. We drop the parent's ``sources`` list deliberately:
-                # the parent already carries those source messages, and chunks
+                # metadata. We pass an empty ``sources`` list deliberately: the
+                # parent already carries those source messages, and chunks
                 # are *derived* fragments — not original messages — so they
                 # should not multiply the full source list (issue #2453, which
                 # produced N byte-identical ``SourceMessage`` copies per chunk
@@ -175,6 +175,7 @@ def _create_chunk_item(chunk_idx: int, chunk):
                     memory_type=item.metadata.memory_type,
                     tags=item.metadata.tags or [],
                     key=item.metadata.key,
+                    sources=[],
                     background=item.metadata.background or "",
                     need_embed=False,
                 )

tests/mem_reader/test_split_sources_no_duplication.py

@@ -109,17 +109,6 @@ def test_split_chunks_preserve_provenance_via_internal_info(self):
             self.assertEqual(chunk.metadata.internal_info["chunk_total"], 2)
             self.assertIn("ingest_batch_id", chunk.metadata.internal_info)
 
-    def test_split_with_no_parent_sources_yields_no_sources(self):
-        """Sanity: a parent without sources still produces sources-less
-        chunks (no spurious empty list creation)."""
-        parent = _build_parent_item([])
-        reader = _build_reader(["chunk 1"])
-
-        chunks = reader._split_large_memory_item(parent, max_tokens=512)
-
-        self.assertEqual(len(chunks), 1)
-        self.assertEqual(chunks[0].metadata.sources, [])
-
 
 if __name__ == "__main__":
     unittest.main()