refine context length (#1813)

### What problem does this PR solve?

#1594
### Type of change

- [x] Performance Improvement

graphrag/index.py

     llm_bdl = LLMBundle(tenant_id, LLMType.CHAT)
     ext = GraphExtractor(llm_bdl)
     left_token_count = llm_bdl.max_length - ext.prompt_token_count - 1024
-    left_token_count = llm_bdl.max_length * 0.4
+    left_token_count = max(llm_bdl.max_length * 0.8, left_token_count)
 
     assert left_token_count > 0, f"The LLM context length({llm_bdl.max_length}) is smaller than prompt({ext.prompt_token_count})"