Refactor react_agent.py to improve security and maintainability

[Mirza-Samad-Ahmed-Baig]

• Removed Hardcoded Credentials: I've replaced the hardcoded OpenAI API key and base URL with environment
  variables to prevent security vulnerabilities.
• Improved Input Validation: I've added checks to prevent the application from crashing due to missing data
  in the input.
• Refactored for Readability: I've broken down the complex _run method into smaller, more manageable
  functions to improve code readability and maintainability.

[Mehrdadm2s]

https://patch-diff.githubusercontent.com/raw/Alibaba-NLP/DeepResearch/pull/143.patch

[Mirza-Samad-Ahmed-Baig]

On running that, I am getting uncountable issues. I tried to solve those all, but they require too much time and solving only some will break the things.