refactor!: remove persistState from IRequestLoader interface

[harryautomazione]

• Closes Remove persistState from IRequestList #3721
• Follow-up to refactor!: Finalize transition to IRequestManager and IRequestLoader #3719
• Removed persistState?(): Promise<void> from IRequestLoader interface (and IRequestManager), as loader implementations handle state persistence internally by listening to EventType.PERSIST_STATE events (e.g. via RecoverableState or EventManager).
• Updated validators.requestList schema in validators.ts to validate ['fetchNextRequest', 'isFinished'] instead of requiring persistState.
• Duck-typed persistState invocation in ThrottlingRequestManager.persistState().
• Updated @crawlee/core public API snapshot in docs/public-api/crawlee-core.api.md.

[B4nan]

heads up: the v4 branch was merged into master (v3 development moved to the 3.x branch), so I rebased this branch onto master and force-pushed it to your fork. Your commits are unchanged, just replayed on the new base. Before adding more changes, reset your local branch to the remote one (e.g. git fetch && git reset --hard @{u}), otherwise you'd reintroduce the old history.