fix: disable job asset actions when preview output is missing (#8700)

### Motivation
- Prevent context-menu actions from operating on stale or non-existent
assets when a completed job has no `previewOutput`, since `Inspect
asset`, `Add to current workflow`, and `Download` should be inactive in
that case.
- Also ensure the `Inspect asset` entry is disabled when the optional
inspect callback is not provided to avoid unexpected behavior.

### Description
- Added an optional `disabled?: boolean` field to the `MenuEntry` type
returned by `useJobMenu` and computed `hasPreviewAsset` to detect when
`taskRef.previewOutput` is present.
- Mark `inspect-asset`, `add-to-current`, and `download` entries as
`disabled` when the preview is missing (and also when the inspect
callback is missing for `inspect-asset`), and keep `delete` omitted when
no preview exists.
- Updated `JobContextMenu.vue` to pass `:disabled="entry.disabled"` to
the rendered `Button` and to short-circuit the action emit when an entry
is disabled.
- Expanded `useJobMenu` unit tests to assert enabled states when a
preview exists and disabled states when preview or inspect handler is
missing.

### Testing
- Ran `pnpm vitest src/composables/queue/useJobMenu.test.ts` and all
tests passed (36 tests).
- Ran `pnpm lint` and the linter reported no warnings or errors.
- Ran `pnpm typecheck` (`vue-tsc --noEmit`) and type checking completed
without errors.

------
[Codex
Task](https://chatgpt.com/codex/tasks/task_e_69864ed56e408330b88c3e9def1b5fb5)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8700-fix-disable-job-asset-actions-when-preview-output-is-missing-2ff6d73d365081b8b72ccadf0ae43e9d)
by [Unito](https://www.unito.io)

src/components/queue/job/JobContextMenu.vue

@@ -26,6 +26,7 @@
           variant="textonly"
           size="sm"
           :aria-label="entry.label"
+          :disabled="entry.disabled"
           @click="onEntry(entry)"
         >
           <i
@@ -68,6 +69,7 @@ function hide() {
 }
 
 function onEntry(entry: MenuEntry) {
+  if (entry.kind === 'divider' || entry.disabled) return
   emit('action', entry)
 }