fix(core): enforce TaskFieldValidatorRegistry validation in CreateTaskUseCase

[amir-rezaei]

Description

This PR resolves issue #1134 by enforcing field validation via TaskFieldValidatorRegistry in CreateTaskUseCase.

Details

• Previously, CreateTaskUseCase bypassed the validator registry entirely when creating new tasks, allowing invalid inputs (e.g., past deadlines, non-positive durations, invalid priorities) to enter the system upon creation even though UpdateTaskUseCase rejected them.
• This PR updates CreateTaskUseCase to instantiate TaskFieldValidatorRegistry and validate input fields (priority, deadline, planned_start, planned_end, estimated_duration) before repository creation.
• Updates FieldValidator, DateTimeValidator, and TaskFieldValidatorRegistry type annotations and logic to support optional task parameter when validating new task creation.

[Kohei-Wada]

Thanks for the contribution, but CI is failing on this PR (Type Check, Test (taskdog-core), E2E), and no fix has been pushed.

Closing to keep the review queue actionable. Please feel free to reopen once:

• all required checks pass on your branch, and
• the change is covered by tests that pass locally (make check && make test).

A PR that fails its own package's test suite can't be reviewed on its merits, so please verify locally before opening.