Add Resilient Fan-Out sample: partial-failure tolerance in a heterogeneous DAG

[brainlimitexceeded]

Summary

Adds a sample demonstrating a pattern I couldn't find covered by the existing Child Workflow docs/samples: a fan-out with one critical branch and several best-effort branches, where:

• Each branch is awaited independently (not via Promise.allOf(...)), so a best-effort failure can't take down a healthy critical branch, or vice versa.
• Still-running best-effort children are explicitly cancelled via a CancellationScope when the critical branch fails, instead of relying on the default ParentClosePolicy (TERMINATE) — TERMINATE delivers no CanceledFailure, so a child that's already dispatched work to a downstream system gets no chance to react before that work runs on, detached, with nowhere to report back to.
• Failure is reconciled via Saga with parallel compensation, repurposed for independent, isolated notifications rather than classic "undo what I did" semantics.

The existing Fan-Out pattern doc covers homogeneous fan-out (identical chunks, all-or-nothing). This sample covers the heterogeneous case with partial-failure tolerance, which seems like a natural follow-up.

What's included

• OnboardingWorkflow + three child workflows (one critical, two best-effort)
• A runnable Starter covering all three outcomes (happy path, degraded, critical failure)
• Unit tests for all three outcomes, including verifying reconciliation activities fire independently on critical failure

Test plan

• ./gradlew :core:test --tests "io.temporal.samples.resilientfanout.OnboardingWorkflowTest" — 3/3 passing
• ./gradlew spotlessCheck — clean

Happy to adjust scope/naming/domain if this isn't the direction you'd want for this pattern — opening this as a starting point for discussion.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}