Let the workflow own a webform interaction's post-submit behavior

When a webform is an interaction step, the workflow now controls what happens after submit, not the webform's own confirmation. On a successful resume the handler returns the party to the interaction's return URL (the dispatcher for a bearer step, the inbox for a task), so the dispatcher renders whatever the run parked on next: a read-only receipt, the next form, a message, or the finished page. This lets one workflow chain several forms, with the same form reusable across steps regardless of its own confirmation settings.

Gated on a submission bound to an Orchestra instance, so standalone webforms are untouched, and the return is honored only for an internal path (no open redirect). Adds coverage: a bearer view-mode submission rendered read-only, and a bearer collect chaining straight into that read-only view.

Closes #3607206