DynamicSymbolLoader.create_component_instance only handled root
schematics: it located its insertion point via `(sheet_instances`,
which exists only in the root .kicad_sch. Adding a component to any
hierarchical sub-sheet raised "Could not find insertion point in
schematic" and aborted the call.
Fall back to inserting just before the closing paren of the outer
(kicad_sch ...) form when the marker is absent. WireManager.add_label
already uses the same fallback for hierarchical sub-sheets.
Adds three regression tests in TestCreateComponentInstanceSubSheet
covering successful insertion, paren balance, and sexpdata round-trip
on a sub-sheet without (sheet_instances).
9843d75c91