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/useResultGallery.ts

@@ -1,39 +1,42 @@
 import { ref, shallowRef } from 'vue'
 
 import type { JobListItem } from '@/composables/queue/useJobList'
-
-/** Minimal preview item interface for gallery filtering. */
-interface PreviewItem {
-  url: string
-  supportsPreview: boolean
-}
-
-/** Minimal task interface for gallery preview. */
-interface TaskWithPreview<T extends PreviewItem = PreviewItem> {
-  previewOutput?: T
-}
+import { findActiveIndex, getOutputsForTask } from '@/services/jobOutputCache'
+import type { ResultItemImpl, TaskItemImpl } from '@/stores/queueStore'
 
 /**
  * Manages result gallery state and activation for queue items.
  */
-export function useResultGallery<T extends PreviewItem>(
-  getFilteredTasks: () => TaskWithPreview<T>[]
-) {
+export function useResultGallery(getFilteredTasks: () => TaskItemImpl[]) {
   const galleryActiveIndex = ref(-1)
-  const galleryItems = shallowRef<T[]>([])
+  const galleryItems = shallowRef<ResultItemImpl[]>([])
 
-  const onViewItem = (item: JobListItem) => {
-    const items: T[] = getFilteredTasks().flatMap((t) => {
-      const preview = t.previewOutput
-      return preview && preview.supportsPreview ? [preview] : []
-    })
+  async function onViewItem(item: JobListItem) {
+    const tasks = getFilteredTasks()
+    if (!tasks.length) return
+
+    const targetTask = item.taskRef
+    const targetOutputs = targetTask
+      ? await getOutputsForTask(targetTask)
+      : null
+
+    // Request was superseded by a newer one
+    if (targetOutputs === null && targetTask) return
+
+    // Use target's outputs if available, otherwise fall back to all previews
+    const items = targetOutputs?.length
+      ? targetOutputs
+      : tasks
+          .map((t) => t.previewOutput)
+          .filter((o): o is ResultItemImpl => !!o)
 
     if (!items.length) return
 
     galleryItems.value = items
-    const activeUrl: string | undefined = item.taskRef?.previewOutput?.url
-    const idx = activeUrl ? items.findIndex((o) => o.url === activeUrl) : 0
-    galleryActiveIndex.value = idx >= 0 ? idx : 0
+    galleryActiveIndex.value = findActiveIndex(
+      items,
+      item.taskRef?.previewOutput?.url
+    )
   }
 
   return {