…#54)

* Fix scheduleTask DateTime contract & restore tests

- TaskScheduler::scheduleTask now converts the \DateTime returned by
  Cron\CronExpression::getNextRunDate() to \DateTimeImmutable before
  calling TaskExecutionRepositoryInterface::create(), which contractually
  requires \DateTimeImmutable. Without this, every cron-driven schedule
  fails with a TypeError when the repository enforces the parameter type.
- TaskExecution::getResult() returns null when the result column was
  never set, instead of false (unserialize(null) === false in PHP 8).
- Repair pre-existing syntax errors ("new \\DateTimeImmutable))" and
  "new \\DateTimeImmutable)'...')") that were left in the test suite by
  the sed-based DateTime → DateTimeImmutable migration in #52, which
  made the whole tests/Unit/Execution and tests/Unit/Storage suites
  un-loadable.
- Update remaining tests that still called the removed findAll() method
  to use findAllPaginated(1).

* Update CI runner to ubuntu-22.04

ubuntu-20.04 has been retired; jobs hang waiting for a runner. See:
https://github.blog/changelog/2025-01-15-github-actions-ubuntu-20-runner-image-brownout-dates-and-other-breaking-changes/

* Address Copilot review

- TaskExecutionInterface::getResult() PHPDoc now documents the null
  return possibility introduced by the unset-result fix.
- Rename leftover testFindAll() methods to testFindAllPaginatedUnbounded()
  so the test name matches the API being exercised.

* Stop tracking phpunit result cache

Accidentally included in the previous commit; add it to .gitignore.