(Fix) API chat controller json return types

[Roardom]

Makes static analysis happy.

[what-the-diff]

PR Summary

• Consistent JSON Response Format
  Updated the return types of several methods to 'JsonResponse' to guarantee uniformity in the format of responses.
• Explicit JSON Responses
  Directly specifying JSON responses instead of general ones to increase clarity of our code.
• Simplification of Methods
  Eliminated the need for comments related to automatic JSON conversion, making our method implementations simpler and cleaner.