fix(forge, agent): Resolve several issues for Chinese language & region

[WangShaoyu1]

update: 1、add response header Content-Type:application/json,charest=UT…F-8, solve cannot display chinese word

2、update the openai api key pattern, like sk-wa-xxx

3、add Optional for additional_input, in TaskRequestBody and StepRequestBody, to avoid 422 status in api

Background

1、cannot show the Chinese word in the fluuter web subproject. Add response data header Content-Type:charset-UTF-8 2、I get a new openAI key,like sk-wa-xxxx, it can work in the chatgpt page ,but cannot be Verified in our project 3、In the flutter for web project，you enter the task in the chat Input, the api will return 422 status, because that the additional_input can not be null, so I add Optional in the file: forge/agent_protocol/models/task.py

PR Quality Scorecard ✨

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• [ ] Have you changed or added a feature?   -4 pts
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  • [ ] Have you added/updated corresponding integration tests?   +5 pts
• [ ] Have you changed the behavior of AutoGPT?   -5 pts
  • [ ] Have you also run agbenchmark to verify that these changes do not regress performance?   +10 pts