fix: respect cancellation after task action returns

[wangedmund77-cmyk]

/claim #1

Summary

• Treat a context cancellation that happens during a task action as a failed task instead of a successful completion.
• Add a regression test where the task action cancels the context and returns nil.

Verification

• Not run locally: Go toolchain is not installed in this environment (go: command not found).
• The change is scoped to cancellation handling and the new test documents the expected final state.

[opirebot]

👀 We've notified the reward creators here.
Make sure your payment account is ready to receive the payment for your hard work 💪