Preserve sub-orchestration parent link across ContinueAsNew

[pinodeca]

Preserves the parent link when a sub-orchestration rolls over with ContinueAsNew, so the logical child can still notify its parent when a later execution completes or fails.

Summary:

• Derive ContinueAsNew parent linkage from the previous execution history in WorkItemReader.
• Preserve the same link in the dispatcher poison/new-history path.
• Add an e2e regression where a parent awaits a child that continues-as-new multiple times before completing.

Validation:

• cargo fmt
• cargo nt -E 'test(test_workitem_reader_with_can) | test(test_workitem_reader_can_preserves_parent_link_from_history) | test(test_workitem_reader_can_carry_forward_prepended) | test(sample_sub_orchestration_continue_as_new_fs)'

[affandar]

Can we wire this into WorkItem::ContinueAsNew event directly instead of pulling from the history?
Needs to be backcompat though.

[pinodeca]

Done in 4f9d29c. The parent link (parent_instance/parent_id) is now stamped directly onto WorkItem::ContinueAsNew by the CAN producer in execution.rs, and the consumers read it from the work item.

For backcompat, both new fields are #[serde(default)] + skip_serializing_if, so old enqueued messages deserialize cleanly and the wire format is unchanged. To stay correct during rolling upgrades (a CAN enqueued by an older node that didn't stamp the fields), the consumers fall back to deriving the link from the previous execution's history when the fields are absent. Added test_workitem_reader_can_preserves_parent_link_from_work_item for the direct path and kept ..._from_history for the fallback path.