Issue #3622715: Give the return target and each doorway's task argument one name

Three request-argument names that are a contract between a writer and a reader were each written out independently. Each now has one declaration, referenced from both ends.

• CommentForm::RETURN_ARG is gone; OrchestraReturn::ARG is the one name for the return target, and the four literals in UserTaskForm and UserOperationForm read it.
• AssignmentGateway::TOKEN_ARG names the query argument the orchestra_interaction_task routes carry the parked token in, read by the gateway, the pending-action handler and the controller.
• EntityTaskController::TOKEN_ARG does the same for the separate orchestra_content.task route, read by the interaction, the form hooks and the page.

Route parameters spelled orchestra_token are untouched: those match a parameter the routing file declares.

No behaviour change, so the test is a contract test rather than a regression one. InteractionTaskTest::testTheDoorwayReadsBackTheLinksItBuilds drives the writer into the reader: it takes the URLs the gateway and the pending-action handler build, asserts the wire name literally, and feeds the parsed query straight into the controller, asserting it resolves the step. Changing TOKEN_ARG to anything else fails it, which was checked. CommentInteractionTest now pins the comment redirect's two query keys as literals rather than as the constants, so a rename cannot move both sides of that assertion at once.