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.test.ts

@@ -16,7 +16,7 @@ type TestTask = {
   executionTime?: number
   executionEndTimestamp?: number
   createTime?: number
-  workflow?: { id?: string }
+  workflowId?: string
 }
 
 const translations: Record<string, string> = {
@@ -161,7 +161,7 @@ const createTask = (
   executionTime: overrides.executionTime,
   executionEndTimestamp: overrides.executionEndTimestamp,
   createTime: overrides.createTime,
-  workflow: overrides.workflow
+  workflowId: overrides.workflowId
 })
 
 const mountUseJobList = () => {
@@ -305,7 +305,7 @@ describe('useJobList', () => {
     expect(vi.getTimerCount()).toBe(0)
   })
 
-  it('sorts all tasks by queue index descending', async () => {
+  it('sorts all tasks by priority descending', async () => {
     queueStoreMock.pendingTasks = [
       createTask({ promptId: 'p', queueIndex: 1, mockState: 'pending' })
     ]
@@ -360,13 +360,13 @@ describe('useJobList', () => {
         promptId: 'wf-1',
         queueIndex: 2,
         mockState: 'pending',
-        workflow: { id: 'workflow-1' }
+        workflowId: 'workflow-1'
       }),
       createTask({
         promptId: 'wf-2',
         queueIndex: 1,
         mockState: 'pending',
-        workflow: { id: 'workflow-2' }
+        workflowId: 'workflow-2'
       })
     ]