Update examples with GoogleGenerativeAI and DeepSeek

• For the GoogleGenerativeAI example: there is some redundant code and since there is no need to explicitly pass in api_key for GoogleGenerativeAI, simply loading GOOGLE_API_KEY into the env vars would be sufficient (reference: https://python.langchain.com/api_reference/google_genai/chat_models/langchain_google_genai.chat_models.ChatGoogleGenerativeAI.html)
• For the DeepSeek examples: add the missing logic of loading api_key