Rename InteractionContext::returnUrl() to nextStepUrl() (it is a continuation URL, not a return URL)

InteractionContext::returnUrl() and branchReturnUrl() are named misleadingly. They do not send the visitor back to where they came from; they build the dispatcher continuation URL (a signed, capability-token link to the orchestra_interaction.step route) that advances the run to whatever it parks on next. In other words they are "continue to the next step" URLs, not "return" URLs.

The name overloads the word "return", which collides with the operator return-to-trace being added in #3610524 (the orchestra_return target). Renaming the continuation methods to say what they do removes the overload: afterwards "return" means only the operator return, and "next step" means workflow chaining.

Proposed change

• Rename InteractionContext::returnUrl() to nextStepUrl().
• Rename InteractionContext::branchReturnUrl() to branchNextStepUrl().
• Update the callers: PaymentInteraction (three uses), ReviewInteractionBase, CommentInteraction, RedirectInteraction (the branch variant), and the SignalOutcomeInteraction test double, plus the OperationGateway comment.
• Update the method docblocks to describe the next-step semantics.

This is a mechanical rename with no behavior change. The query argument names those methods feed (a downstream form's return field) are handled separately in #3610524; this issue is only the method naming on InteractionContext.