PDJB-896: Merge Task and DuplicableTaskWithDependencies

[AEPR]

Ticket number

PDJB-896

Goal of change

Fold DuplicableTaskWithDependencies into Task now that every task in the codebase owns its own state, so the journey framework exposes a single concrete task base rather than three.

Description of main change(s)

• Task<TState, TDependencies> becomes the merged base — always route-scoped, self-stated via AbstractJourneyState, with an optional typed dependencies reference to the enclosing state.
• The zero-dependencies convenience subclass DuplicableTask is renamed to TaskWithoutDependencies (after the merge, "duplicable" no longer distinguishes anything — every task is).
• Collapses the redundant DSL split: duplicableTask { } → task { } and duplicableCheckAnswerTask → checkAnswerTask (the previously unused bare-Task overloads are deleted).
• Updates ~45 call sites across journeys/** and 11 task-subclass supertypes; consolidates DuplicableTaskTests + DuplicableTaskWithDependenciesTests into TaskTests with three nested groupings; introduces OccupationTask.inJourney(journey) (called at the mount site) so the legacy task's taskState get() = dependencies binding is safe against eager evaluation by the DSL.
• Behaviour-preserving refactor. No user-visible change, no migrations, no feature flag.

Anything you'd like to highlight to the reviewer?

OccupationTask.inJourney(journey) is a legacy shim needed because that task's taskState get() = dependencies would otherwise be evaluated by the DSL before withDependencies { } runs. It's called at the four mount sites and will be removed with OccupationTask itself under PDJB-1340.

Checklist

• Unit tests for new logic (e.g. new service methods) have been added
• Test suite has been run in full locally and is passing
• Branch has been rebased onto main and run locally, with everything working as expected (both for your new feature and any related functionality)
• TODO comments referencing this JIRA ticket have been searched for and removed - if a future PR will address them, mention that here
• This feature is not behind a feature flag. I've checked that this is appropriate and we're happy with this code becoming live as soon as we release