frappe.get_list defaults to 20 rows; child-table joins can produce
duplicate parent names that fill the window and hide further drafts.
Also clarify the check-ordering regression test.
When creating a follow-up document (SO->DN, PO->PR, PI->Payment Entry,
etc.), warn the user if a draft of the target doctype already linked to
the source document exists, with links to the drafts and the option to
proceed anyway.
The target doctype comes from the make_mapped_doc response via the new
frappe.model.add_mapped_doc_guard hook, so every open_mapped_doc flow is
covered without per-doctype code or method-name inference. The server
lookup walks parent-level and child-table Link / Dynamic Link fields of
the target doctype and queries through frappe.get_list, so role and user
permissions apply and docstatus filtering happens in the query itself.
Payment Entry creation bypasses open_mapped_doc, so its controller runs
the same guard explicitly.
