Fix: Disable reasoning on Gemini 2.5 Flash by default (#10477)

### What problem does this PR solve?

Gemini 2.5 Flash Models use reasoning by default. There is currently no
way to disable this behaviour. This leads to very long response times (>
1min). The default behaviour should be, that reasoning is disabled and
configurable

issue #10474 

### Type of change

- [X] Bug Fix (non-breaking change which fixes an issue)

rag/llm/chat_model.py

@@ -1188,8 +1188,36 @@ class GoogleChat(Base):
                     del gen_conf[k]
         return gen_conf
 
+    def _get_thinking_config(self, gen_conf):
+        """Extract and create ThinkingConfig from gen_conf.
+
+        Default behavior for Vertex AI Generative Models: thinking_budget=0 (disabled)
+        unless explicitly specified by the user. This does not apply to Claude models.
+
+        Users can override by setting thinking_budget in gen_conf/llm_setting:
+        - 0: Disabled (default)
+        - 1-24576: Manual budget
+        - -1: Auto (model decides)
+        """
+        # Claude models don't support ThinkingConfig
+        if "claude" in self.model_name:
+            gen_conf.pop("thinking_budget", None)
+            return None
+
+        # For Vertex AI Generative Models, default to thinking disabled
+        thinking_budget = gen_conf.pop("thinking_budget", 0)
+
+        if thinking_budget is not None:
+            try:
+                import vertexai.generative_models as glm  # type: ignore
+                return glm.ThinkingConfig(thinking_budget=thinking_budget)
+            except Exception:
+                pass
+        return None
+
     def _chat(self, history, gen_conf={}, **kwargs):
         system = history[0]["content"] if history and history[0]["role"] == "system" else ""
+        thinking_config = self._get_thinking_config(gen_conf)
         gen_conf = self._clean_conf(gen_conf)
         if "claude" in self.model_name:
             response = self.client.messages.create(
@@ -1223,6 +1251,9 @@ class GoogleChat(Base):
                     }
                 ]
 
+        if thinking_config:
+            response = self.client.generate_content(hist, generation_config=gen_conf, thinking_config=thinking_config)
+        else:
             response = self.client.generate_content(hist, generation_config=gen_conf)
         ans = response.text
         return ans, response.usage_metadata.total_token_count
@@ -1255,6 +1286,7 @@ class GoogleChat(Base):
             response = None
             total_tokens = 0
             self.client._system_instruction = system
+            thinking_config = self._get_thinking_config(gen_conf)
             if "max_tokens" in gen_conf:
                 gen_conf["max_output_tokens"] = gen_conf["max_tokens"]
                 del gen_conf["max_tokens"]
@@ -1272,6 +1304,9 @@ class GoogleChat(Base):
                     ]
             ans = ""
             try:
+                if thinking_config:
+                    response = self.client.generate_content(history, generation_config=gen_conf, thinking_config=thinking_config, stream=True)
+                else:
                     response = self.client.generate_content(history, generation_config=gen_conf, stream=True)
                 for resp in response:
                     ans = resp.text