Basic tests

Add minimal tests for `POST /tasks` validation and scheduler selection logic.

Look for the route handling POST /tasks, likely in a file like src/routes/tasks.ts or similar. Find the validation logic and the scheduler selection. Write tests for these using the project's testing framework (probably Jest or Mocha). Check existing test files for patterns. Running the test suite will confirm the new tests pass.

索引モデルが issue の本文から書いたものです。