Add test coverage for validateDateRange function

## Description

The newly implemented `validateDateRange` function in `src/device-registry/validators/activities.validators.js` lacks test coverage. This function is critical as it validates that activity dates are not in the future and not more than one month in the past.

## Requirements

Please add unit tests that verify:
- A date in the future correctly triggers an error.
- A date more than one month in the past correctly triggers an error.
- A valid date within the acceptable range returns `true`.

## Context
- Referenced in PR: #4515
- Comment URL: https://github.com/airqo-platform/AirQo-api/pull/4515#discussion_r1977414281

## Importance
Adding these tests will help ensure robust edge-case handling and maintain code quality as the application evolves.

## Acceptance Criteria
- [ ] Unit tests created for the `validateDateRange` function
- [ ] All test cases passing
- [ ] Code coverage for the function reaches 100%