Fix when_all to wait for all tasks before surfacing failures

[andystaples]

Summary

task.when_all() previously failed fast: as soon as one of its child tasks failed, the composite task was marked complete and the failure was surfaced immediately, even while sibling tasks were still running.

This does not match the .NET SDK (durabletask-dotnet), which relies on the framework's Task.WhenAll. That contract is: the returned task completes only after all supplied tasks finish, and then surfaces the first fault.

This PR changes WhenAllTask.on_child_completed to record the first exception but only complete once every child task has completed — surfacing the first failure at that point (or building the ordered result list if none failed).

Behavior change

An orchestration that fans out and hits a failure now waits for the remaining sibling tasks to finish before failing, instead of failing immediately. This is a deliberate change to match .NET semantics.

Changes

• durabletask/task.py — WhenAllTask.on_child_completed no longer sets _is_complete on the first failure; it completes (with the first exception, if any) only after all children complete.
• tests/durabletask/test_orchestration_executor.py — updated test_fan_in_with_single_failure to assert the orchestration keeps running (zero actions) while tasks are outstanding, then fails once all children complete.
• CHANGELOG.md — added a FIXED entry.

Testing

• pytest tests/durabletask/test_orchestration_executor.py — all pass
• flake8 clean on changed files

[Copilot]

Pull request overview

This PR adjusts the core task.when_all() composite-task behavior in the Durable Task Python SDK to match .NET Task.WhenAll semantics by waiting for all child tasks to finish before completing and surfacing failures.

Changes:

• Updated WhenAllTask.on_child_completed to delay completion until every child task finishes, recording the first failure to surface at the end.
• Updated the fan-in unit test to assert the orchestration keeps running (no new actions) after an early failure until remaining tasks complete.
• Added an Unreleased changelog entry documenting the behavior change.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
durabletask/task.py	Updates WhenAllTask completion/failure timing to avoid fail-fast behavior.
tests/durabletask/test_orchestration_executor.py	Adjusts test expectations to confirm orchestration remains running until all children complete, then fails.
CHANGELOG.md	Documents the when_all() behavioral fix under Unreleased / FIXED.