Add health check endpoint with handlers, routes, and unit tests for all node types

[Sophie-Ag00001]

# Add Health Check Endpoint

## Summary
This PR introduces a health check endpoint to the API. The changes include:
- Adding a `health_check` function in `src/api/handlers.rs`.
- Updating routes in `src/api/routes.rs` for each node type (User, Storage, Mempool, Miner, Miner+User) to include the health check endpoint.
- Adding a unit test for the `/health` route.

## Motivation
The health check endpoint is essential for monitoring the status of the service. It provides a simple way to verify that the service is operational and healthy.

## Context
The health check endpoint returns a straightforward response indicating the service is healthy. This is a common practice in microservices and distributed systems to ensure reliability and ease of monitoring.

## Testing
To test the changes:
1. Run the unit tests using `cargo test`.
2. Manually verify the `/health` endpoint by sending a request to it and ensuring it returns the expected response.

## Known Issues
- None at this time.