fix: remove response_format=json_object from chat_json, increase ontology max_tokens

Bug 1: chat_json() was passing response_format={'type': 'json_object'}
to the LLM, which enforces JSON grammar from token 0. Reasoning
models (Qwen3, DeepSeek-R1, etc.) generate <think>...</think> blocks
before JSON output, causing garbled results. The fix removes the
response_format parameter since the system prompt already requests
JSON output and the existing <think> cleanup handles any remaining
tags.

Bug 2: ontology_generator hardcoded max_tokens=4096, causing
truncation for models with larger context windows. Increased to
16384 to accommodate reasoning model outputs.

Fixes #642

backend/app/services/ontology_generator.py

@@ -217,7 +217,7 @@ class OntologyGenerator:
         result = self.llm_client.chat_json(
             messages=messages,
             temperature=0.3,
-            max_tokens=4096
+            max_tokens=16384
         )
         
         # 验证和后处理

backend/app/utils/llm_client.py

@@ -88,7 +88,6 @@ class LLMClient:
             messages=messages,
             temperature=temperature,
             max_tokens=max_tokens,
-            response_format={"type": "json_object"}
         )
         # 清理markdown代码块标记
         cleaned_response = response.strip()