Add parent_instance_id to OrchestrationContext

[andystaples]

Summary

Adds a parent_instance_id property to OrchestrationContext. For a sub-orchestration, it returns the instance ID of the orchestration that scheduled it; for a top-level orchestration, it returns None.

Changes

• durabletask/task.py — new abstract parent_instance_id property on OrchestrationContext.
• durabletask/worker.py — _RuntimeOrchestrationContext implements the property, backed by _parent_instance_id, populated from executionStarted.parentInstance.orchestrationInstance.instanceId (guarded by HasField).
• tests/durabletask/test_orchestration_executor.py — two new tests: a sub-orchestration exposes its parent's instance ID, and a top-level orchestration returns None.
• CHANGELOG.md — added an entry under ## Unreleased.

Testing

• python -m pytest tests/durabletask/test_orchestration_executor.py — all tests pass, including the two new ones.
• flake8 clean on all changed files; no Pylance/type errors.
• markdownlint clean on the changelog additions.

[Copilot]

Pull request overview

Adds a new parent_instance_id property to the Durable Task Python SDK’s OrchestrationContext, enabling orchestrator code to detect whether it is running as a sub-orchestration and, if so, identify the parent orchestration instance.

Changes:

• Added OrchestrationContext.parent_instance_id as a new abstract property returning str | None.
• Implemented the property in the runtime orchestration context and populated it from executionStarted.parentInstance.orchestrationInstance.instanceId when present.
• Added tests covering both sub-orchestration and top-level orchestration behavior, and documented the new API in CHANGELOG.md.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
durabletask/task.py	Introduces the new abstract parent_instance_id API on OrchestrationContext.
durabletask/worker.py	Implements and populates parent_instance_id in _RuntimeOrchestrationContext during executionStarted processing.
tests/durabletask/test_orchestration_executor.py	Adds unit tests validating parent_instance_id for sub-orchestrations and top-level orchestrations.
CHANGELOG.md	Documents the new OrchestrationContext.parent_instance_id property under ## Unreleased.