Add test coverage for Timetable.load method

[coderabbitai]

Description

The Timetable.load method in supertask/model.py has branches for JSON and YAML loading as well as error handling for unsupported file types, but these aren't covered by tests.

Requirements

Please add tests for:

• Loading a JSON file via Timetable.load(…) and verifying the returned data
• Loading a .yml file (if treated differently than .yaml)
• Passing an unsupported extension (e.g. .txt) and asserting that NotImplementedError is raised

Reference

• PR: Refactor data model and CLI. Remove HTTP service. #155
• Comment: Refactor data model and CLI. Remove HTTP service. #155 (comment)

Technical Details

The method is located at lines 181-189 in supertask/model.py. Current tests only exercise the YAML branch.

Suggested locations to add tests:

• tests/test_core.py or a new file under tests/
• Consider using temporary files or fixtures for JSON/.yml content and an invalid-extension case.