Add ability to invoke the ParentClosePolicy on chain completion not the run completion.

Is your feature request related to a problem? Please describe.
When a parent calls continue-as-new the child behaves according to its ParentClosePolicy. A common scenario is to keep children running until the whole chain of parent continue-as-new calls is complete. In this case the only option is to set ParentClosePolicy to ABANDON. But it disables the ability to terminate all the children automatically when the parent is terminated.

Describe the solution you'd like
Add a flag that specifies if ParentClosePolicy applies when the chain is done or the run is done. Besides not being backward compatible, I propose to default it to when the whole chain is done. The strawman for the flag is parentClosePolicyScope with RUN, WORKFLOW_CHAIN (or just WORKFLOW).

If parentClosePolicyScope is set to WORKFLOW_CHAIN then terminating the last run of a workflow will terminate all the children started by the whole chain. This obviously should obey the parent retention interval as information about the past children started by deleted due to retention runs might not be available.

The issue names no files, tests, or entry points. Start by locating the ParentClosePolicy handling and continue-as-new chain lifecycle logic, then define how RUN and WORKFLOW_CHAIN scopes interact with termination and retention. Done means the selected scope is supported without breaking existing behavior and the requested child-termination semantics are verified.