[sample-python]: Add a FastAPI + MongoDB sample for test suite using Keploy API Test Generator #3599

Description:

We should add a complete FastAPI + MongoDB sample that demonstrates generating and running an API test suite using Keploy’s API Test Generator. The goal is to provide a ready-to-run reference implementation that shows how Keploy captures requests/responses against a Mongo-backed FastAPI app and produces reproducible tests for CRUD endpoints .