Standalone activity: clean up outcome computation

[dandavison]

What changed?

Cleaner implementation of logic involved when obtaining activity outcome for DescribeActivityExecution and PollActivityExecution.

Why?

Code cleanup. Suggested at #8771 (comment)

How did you test it?

• built
• covered by existing tests

---

Note

Streamlines outcome to return nil unless closed and consolidates failure resolution using last attempt details.

• Activity execution outcome (chasm/lib/activity/activity.go):
  • Add closed-state guard in outcome to return nil unless lifecycle is closed.
  • Simplify outcome resolution: prefer Outcome.Successful/Outcome.Failed; otherwise fall back to LastAttempt.LastFailureDetails (removes status-based checks).

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