Fix WhenAllTask constructor resetting _completedTasks counter

[YunchuWang]

Problem

Fixes #131

The WhenAllTask constructor resets _completedTasks to 0 after calling super(tasks). The CompositeTask base constructor processes already-complete children via onChildCompleted(), correctly incrementing _completedTasks. The subsequent reset in WhenAllTask discards this count, causing the task to never complete when some children are already complete at construction time.

Root Cause

In when-all-task.ts, lines 14-15:

constructor(tasks: Task<T>[]) {
    super(tasks);           // CompositeTask processes pre-completed children → _completedTasks = K
    this._completedTasks = 0; // ← BUG: resets counter to 0
    this._failedTasks = 0;    // ← Dead code: never incremented anywhere
}

When WhenAllTask is constructed with a mix of already-complete and pending tasks:

1. CompositeTask correctly counts the already-complete children (e.g., _completedTasks = K)
2. WhenAllTask resets _completedTasks = 0
3. When the remaining N - K tasks complete, _completedTasks reaches N - K instead of N
4. The completion check _completedTasks == _tasks.length is never true
5. The WhenAllTask never completes, causing the orchestration to hang

Fix

Removed the redundant this._completedTasks = 0 and this._failedTasks = 0 lines. The base class CompositeTask already initializes both fields to 0 before processing pre-completed children.

Tests

Added 8 unit tests in when-all-task.spec.ts:

• Empty task array completion
• All pending children completing normally
• Fail-fast on child failure
• Pre-completed children (the bug scenario from [copilot-finds] Bug: WhenAllTask constructor resets _completedTasks counter, causing hang with pre-completed children #131)
• All children pre-completed at construction
• Pre-failed child detection
• Post-fail-fast completion (no double-complete)
• Pending tasks count tracking

All 839 unit tests pass.

[Copilot]

Pull request overview

This PR fixes issue #131 where WhenAllTask's constructor reset _completedTasks to 0 after calling super(tasks), which caused WhenAllTask to never complete when constructed with pre-completed children. The CompositeTask base class constructor correctly initializes counters to 0 and then processes pre-completed children via onChildCompleted(). The redundant re-initialization in the subclass wiped out that count.

Changes:

• Removed the redundant this._completedTasks = 0 and this._failedTasks = 0 lines from the WhenAllTask constructor and added an explanatory comment
• Added a comprehensive test file (when-all-task.spec.ts) with 8 unit tests covering empty arrays, pending children, fail-fast, pre-completed children (the bug scenario), and pending tasks tracking

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
packages/durabletask-js/src/task/when-all-task.ts	Removed redundant counter re-initialization after super() call, added explanatory comment
packages/durabletask-js/test/when-all-task.spec.ts	New test file with 8 unit tests covering WhenAllTask behavior including the regression scenario from #131

[Copilot]

The Task import is unused in this test file — only WhenAllTask and CompletableTask are referenced. It should be removed to keep the imports clean.