refactor: encapsulate error extraction in TaskItemImpl getters (#7650)

## Summary
- Add `errorMessage` and `executionError` getters to `TaskItemImpl` that
extract error info from status messages
- Update `useJobErrorReporting` composable to use these getters instead
of standalone function
- Remove the standalone `extractExecutionError` function

This encapsulates error extraction within `TaskItemImpl`, preparing for
the Jobs API migration where the underlying data format will change but
the getter interface will remain stable.

## Test plan
- [x] All existing tests pass
- [x] New tests added for `TaskItemImpl.errorMessage` and
`TaskItemImpl.executionError` getters
- [x] TypeScript, lint, and knip checks pass

🤖 Generated with [Claude Code](https://claude.com/claude-code)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7650-refactor-encapsulate-error-extraction-in-TaskItemImpl-getters-2ce6d73d365081caae33dcc7e1e07720)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Christian Byrne <cbyrne@comfy.org>

src/composables/queue/useJobList.ts

@@ -238,7 +238,7 @@ export function useJobList() {
       const activeId = workflowStore.activeWorkflow?.activeState?.id
       if (!activeId) return []
       entries = entries.filter(({ task }) => {
-        const wid = task.workflow?.id
+        const wid = task.workflowId
         return !!wid && wid === activeId
       })
     }