[Fix] Task reliability metrics include environment snapshot maintenance

[roomote-roomote]

​Created by Roomote. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

• Exclude background environment snapshot maintenance from task_settled, matching the existing task_created analytics contract.
• Let the centralized terminal run path own failed snapshot state instead of issuing a second worker-side status update.
• Cover snapshot settlement exclusion across completed, failed, and canceled outcomes, plus worker failure delegation.

Why this change was made

Product task reliability could appear worse when background snapshot maintenance failed because snapshot runs were counted only on the settlement side. Failed snapshots also attempted two status transitions, producing misleading conflict errors after the centralized terminal path had already finalized them.

Impact

Task settlement analytics now describe product task outcomes consistently, while failed environment snapshots still reach a terminal failed state through the shared finalization path without duplicate status-write noise.

[roomote-community]

No code issues found. See task

_{Reviewed a6589de}