implement err group as an alternative to a wait group so errors from activities can be propagated upwards nicely

[DerkSchooltink]

implement #461

[Copilot]

Pull request overview

This PR adds a workflow-native ErrGroup (errgroup-style) abstraction so concurrent workflow coroutines can return errors that are aggregated and propagated via Wait, and includes a sample demonstrating the new API.

Changes:

• Expose workflow.ErrGroup and workflow.WithErrGroup as public workflow APIs.
• Implement internal/sync.ErrGroup with tests for success and “first error wins”.
• Add a new sample (samples/concurrent-errgroup) demonstrating concurrent activity execution with error propagation.

Reviewed changes

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

File	Description
workflow/sync.go	Exposes the new ErrGroup type alias and WithErrGroup helper at the workflow API layer.
internal/sync/errgroup.go	Implements the scheduler-compatible errgroup abstraction.
internal/sync/errgroup_test.go	Adds unit tests validating basic ErrGroup behavior.
samples/concurrent-errgroup/concurrent_errgroup.go	Demonstrates usage of workflow.WithErrGroup in a runnable sample.

Comments suppressed due to low confidence (2)

internal/sync/errgroup.go:40

• errGroup has a waiting flag intended for misuse detection, but it’s currently unused beyond being set in Wait. Either remove it to avoid dead state, or use it to enforce a clear contract (e.g., disallow calling Go after Wait starts, similar to WaitGroup).

This issue also appears on line 58 of the same file.

	// track if Wait was called to detect certain misuses (optional)
	waiting bool

internal/sync/errgroup.go:60

• Wait sets waiting=true, but Go does not check it. This allows Go to be called after Wait has started/returned, which can make Wait return without waiting for the newly launched coroutine (i.e., silent misuse). Consider adding a panic/guard in Go when waiting is already true (mirroring WaitGroup’s misuse detection) so incorrect usage fails fast.

func (g *errGroup) Go(f func(Context) error) {
	g.n += 1

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