langchain

6bc6d64a - langchain_google_vertexai[patch]: Add support for SystemMessage for Gemini chat model (#15933)

langchain_google_vertexai[patch]: Add support for SystemMessage for Gemini chat model (#15933) - **Description:** In Google Vertex AI, Gemini Chat models currently doesn't have a support for SystemMessage. This PR adds support for it only if a user provides additional convert_system_message_to_human flag during model initialization (in this case, SystemMessage would be prepended to the first HumanMessage). **NOTE:** The implementation is similar to #14824 - **Twitter handle:** rajesh_thallam --------- Co-authored-by: Erick Friis <erick@langchain.dev>