Refactor: Enhance delta streaming in chat functions for improved reasoning and content handling (#12453)

### What problem does this PR solve?

change:
Enhance delta streaming in chat functions for improved reasoning and
content handling

### Type of change


- [x] Refactoring

api/db/services/conversation_service.py

@@ -69,6 +69,7 @@ def structure_answer(conv, ans, message_id, session_id):
     if not isinstance(reference, dict):
         reference = {}
         ans["reference"] = {}
+    is_final = ans.get("final", True)
 
     chunk_list = chunks_format(reference)
 
@@ -81,12 +82,29 @@ def structure_answer(conv, ans, message_id, session_id):
 
     if not conv.message:
         conv.message = []
+    content = ans["answer"]
+    if ans.get("start_to_think"):
+        content = "<think>"
+    elif ans.get("end_to_think"):
+        content = "</think>"
+
     if not conv.message or conv.message[-1].get("role", "") != "assistant":
-        conv.message.append({"role": "assistant", "content": ans["answer"], "created_at": time.time(), "id": message_id})
+        conv.message.append({"role": "assistant", "content": content, "created_at": time.time(), "id": message_id})
     else:
-        conv.message[-1] = {"role": "assistant", "content": ans["answer"], "created_at": time.time(), "id": message_id}
+        if is_final:
+            if ans.get("answer"):
+                conv.message[-1] = {"role": "assistant", "content": ans["answer"], "created_at": time.time(), "id": message_id}
+            else:
+                conv.message[-1]["created_at"] = time.time()
+                conv.message[-1]["id"] = message_id
+        else:
+            conv.message[-1]["content"] = (conv.message[-1].get("content") or "") + content
+            conv.message[-1]["created_at"] = time.time()
+            conv.message[-1]["id"] = message_id
     if conv.reference:
-        conv.reference[-1] = reference
+        should_update_reference = is_final or bool(reference.get("chunks")) or bool(reference.get("doc_aggs"))
+        if should_update_reference:
+            conv.reference[-1] = reference
     return ans
 
 async def async_completion(tenant_id, chat_id, question, name="New session", session_id=None, stream=True, **kwargs):