Add input validation with Pydantic models

[sweetlife999]

Improve your FastAPI app by creating a new POST endpoint /echo that accepts a JSON object with a message field, and returns the same message back. Use a Pydantic model for input validation. Return an error if the message is missing or not a string.

Steps:

• Create a Pydantic model for input data.
• Add POST /echo endpoint using that model.
• Return input as JSON or an error message for invalid input.

Label this issue as 'good first issue'.