Instance trace: offer an "Open" link to a parked human node's operator surface (task and operation nodes)

On the process-instance trace, a parked human node offers a "Signal" link per outcome and "Fire timer now", but no "Open" link to the node's operator surface, unless the node also declares a visitor-facing interaction block. So an interaction-task or operation node whose only interaction is its operator surface (an order review, an attendance review) shows no way to open it from the trace, even though the same node's "Open" action appears fine on the pending-actions / Orders list.

Cause

The trace's "Open" link came only from the token's has_interaction field, which reflects the visitor interaction block (the bearer dispatcher page), and deliberately ignores an interaction task's operator interaction_plugin. The pending-actions list, by contrast, builds its "open it" URL from the node's pending-action handler (actUrl), which covers every doorway. The two surfaces disagreed.

Fix

• Add PendingActionsFinder::actUrlFor(token): the account-agnostic "Open" URL of a parked human node, resolved through its pending-action handler, the same URL the pending-actions list uses.
• The trace offers an "Open" link whenever actUrlFor is non-NULL, so it works for both task and operation modes uniformly (the target route enforces its own access).
• Keep the visitor/bearer "Open" only for a pure bearer interaction with no operator surface, guarded so the two never double-add.

Test

PendingActionsFinderTest asserts actUrlFor returns the operation page for a parked operation and the inbox completion form for a parked user task, and NULL once the token is no longer parked.