docs(integration-toolkit): relation_refs append to existing targets; entity order is not a processing order

[anttiviljami]

Summary

Public-docs follow-up to integration-toolkit-api MR !252 (STABLE360-13057). Changes to integration-toolkit/inbound/relations.md:

• Entity order: replaces "Process parent entities before children" with "Don't Rely on Entity Order". The entities of one event are processed in parallel (one FIFO message group per entity), so array order is not a processing order. A customer relied on it, and that reliance surfaced the bug.
• Processing flow: if the related entity already exists, the relation_ref value is appended, so the entity's other items are kept whatever value.operation says. value.operation only applies when the related entity is created.
• New section "When the Referenced Item Doesn't Exist Yet":
  • a missing item on an existing entity is appended, then the update is retried;
  • RELATION_REF_ITEM_NOT_FOUND is only raised if the item still doesn't match after its value was written;
  • a tip explains how a _set in the related entity's own mapping churns the items relation_refs appended.

Test plan

• Docs build passes
• Merge only after integration-toolkit-api !252 is deployed to prod

🤖 Generated with Claude Code