Test `/combined-data` endpoint

🎯 Goal / Objective

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Write unit tests for the /combined-data endpoint in server.js.

✅ Tasks to be Completed

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• Test that a successful data fetch returns status 200 and expected data type
• Test that any inputs into the endpoint are properly validated
• Test that bad inputs return status 400
• Test that trying to fetch non-existent data returns status 404
• The above steps are just a place to get started. Test for more edge cases and possible inputs/outputs than the steps listed here.
• Fix any errors that failed tests expose.

Acceptance Criteria

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• The new feature is implemented as described.
• All related code passes the repository's workflow checks (linting, tests).
• The application runs without errors after the changes are made.

Additional Context

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#263 is a prerequisite. See the parent issue for more information.

Read issue #263 first, then inspect the /combined-data endpoint in server.js to identify its inputs, successful response, validation, and not-found behavior. Add tests covering the listed 200, 400, and 404 cases plus relevant edge cases, and finish when the repository workflow checks pass without application errors.