Fix WhenAllTask crash when children complete after fail-fast

[YunchuWang]

Fix #120
WhenAllTask.onChildCompleted() had two bugs:

1. Threw 'Task is already completed' when a child completed after the WhenAllTask had already failed via fail-fast. This crashed orchestrations when multiple activities were in a WhenAll and one failed while others completed in the same or subsequent event batch.

2. Fell through from the fail-fast block to the result-collection block when the failing task was the last child to complete, causing getResult() to throw on the failed task.

Fixes:

• Change throw to return in onChildCompleted when already complete
• Add return after fail-fast to prevent fall-through to getResult()
• Add _isComplete guard in RuntimeOrchestrationContext.resume() to prevent attempting to resume a finished generator

Summary

What changed?

Why is this change needed?

Issues / work items

• Resolves #
• Related #

---

Project checklist

• Release notes are not required for the next release
  • Otherwise: Notes added to CHANGELOG.md
• Backport is not required
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• All required tests have been added/updated (unit tests, E2E tests)
• Breaking change?
  • If yes:
    • Impact:
    • Migration guidance:

---

AI-assisted code disclosure (required)

Was an AI tool used? (select one)

• No
• Yes, AI helped write parts of this PR (e.g., GitHub Copilot)
• Yes, an AI agent generated most of this PR

If AI was used:

• Tool(s):
• AI-assisted areas/files:
• What you changed after AI output:

AI verification (required if AI was used):

• I understand the code and can explain it
• I verified referenced APIs/types exist and are correct
• I reviewed edge cases/failure paths (timeouts, retries, cancellation, exceptions)
• I reviewed concurrency/async behavior
• I checked for unintended breaking or behavior changes

---

Testing

Automated tests

• Result: Passed / Failed (link logs if failed)

Manual validation (only if runtime/behavior changed)

• Environment (OS, Node.js version, components):
• Steps + observed results:
  1.
  2.
  3.
• Evidence (optional):

---

Notes for reviewers

• N/A

[Copilot]

Pull request overview

Fixes a crash in the core task-composition logic (whenAll) by making WhenAllTask resilient to child completions that arrive after the composite has already fail-fast completed, and by preventing the runtime from trying to resume an already-finished orchestrator generator.

Changes:

• Update WhenAllTask.onChildCompleted() to ignore completions after completion, and to return immediately after fail-fast to prevent fall-through into result collection.
• Add an _isComplete guard in RuntimeOrchestrationContext.resume() to avoid resuming finished orchestrations.
• Add regression tests covering fail-fast + late completions and the “last completion is the failing one” scenario.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/durabletask-js/src/task/when-all-task.ts	Prevents fail-fast whenAll from throwing on later child completions and avoids fall-through into getResult() after a failure.
packages/durabletask-js/src/worker/runtime-orchestration-context.ts	Prevents attempts to resume a generator after orchestration completion.
packages/durabletask-js/test/orchestration_executor.spec.ts	Adds targeted regression coverage for whenAll fail-fast edge cases and caught-failure behavior.