Adds retrieval result fields to Chunk (#8478)

### What problem does this PR solve?

This PR adds fields to the `Chunk` class to store retrieval results like
similarity scores, term similarity, vector similarity, positions, and
document type. This allows the chunk object to hold all the information
needed when returning search results from the vector database.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

sdk/python/ragflow_sdk/modules/chunk.py

@@ -35,6 +35,12 @@ class Chunk(Base):
         self.document_name = ""
         self.document_id = ""
         self.available = True
+        # Additional fields for retrieval results
+        self.similarity = 0.0
+        self.vector_similarity = 0.0
+        self.term_similarity = 0.0
+        self.positions = []
+        self.doc_type = ""
         for k in list(res_dict.keys()):
             if k not in self.__dict__:
                 res_dict.pop(k)