fix: handle None from_entity in Context API RelationSummary

Update typedef for RelationSummary

Signed-off-by: Drew Cain <groksrc@gmail.com>

Author: groksrc

src/basic_memory/schemas/memory.py

@@ -134,7 +134,7 @@ class RelationSummary(BaseModel):
     file_path: str
     permalink: str
     relation_type: str
-    from_entity: str
+    from_entity: Optional[str] = None
     to_entity: Optional[str] = None
     created_at: datetime