Add tests for Flask App

There are no tests for the Flask [app.py](https://github.com/aafre/resume-builder/blob/main/app.py) at the moment. We need to set up testing using `pytest` and cover the most critical scenarios.

We can start with simple tests and then extend them later.

The `docker.dev.api` file can be used to run the application locally using Docker. The app needs to run on Linux, hence, Docker is the preferred way to run the app locally. Look at the [local development steps](https://github.com/aafre/resume-builder?tab=readme-ov-file#local-development) to run via Docker.

It is also imperative that we add tests for the [resume_generator.py](https://github.com/aafre/resume-builder/blob/main/resume_generator.py) script, as that is where the core functionality lies and Flask is just an HTTP wrapper on top of it.

Start by reading the local development steps to run the app via Docker using the docker.dev.api file. The main files to test are app.py (Flask HTTP wrapper) and resume_generator.py (core functionality). Set up pytest, write initial tests for critical scenarios in these files, and ensure they pass in the Docker environment.

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