chore: change default model from gemini-1.5-flash to gemini-2.5-flash (#687)

Co-authored-by: Mini256 <minianter@foxmail.com>

Author: chethanuk

backend/app/rag/llms/provider.py

@@ -80,7 +80,7 @@ class LLMProviderOption(BaseModel):
         provider_display_name="Vertex AI",
         provider_description="Vertex AI is a fully-managed, unified AI development platform for building and using generative AI.",
         provider_url="https://cloud.google.com/vertex-ai",
-        default_llm_model="gemini-1.5-flash",
+        default_llm_model="gemini-2.5-flash",
         llm_model_description="Find more in https://cloud.google.com/model-garden",
         credentials_display_name="Google Credentials JSON",
         credentials_description="The JSON Object of Google Credentials, refer to https://cloud.google.com/docs/authentication/provide-credentials-adc#on-prem",