Alert the user when the agent plan query fails

What does this MR do and why?

When the agent plan query moved from work_plan.vue up into work_item_ai_widget.vue (!254260 (merged)), its Apollo error handler only reported to Sentry, so a failed readiness score or plan fetch left the widget silently empty with nothing to tell the user why. This was flagged as a non-blocking follow-up in that review.

This MR shows an alert from the query's error handler, in the same way work_plan.vue surfaces its update errors with createAlert, and keeps the Sentry report. One string is added to locale/gitlab.pot. The spec covers the failure path and fails without the change.

References

• Resolves #628571
• Origin: !254260 (comment 3817317975)

Differences

Before: If the workItemAgentPlan query fails, the AI widget renders with no plan and no message.

After: An alert reads "Something went wrong when fetching the agent plan. Please refresh this page."

How to set up and validate locally

1. Open a work item that shows the AI widget.
2. Block the workItemAgentPlan GraphQL request in the browser's Network tab and reload.
3. On master the widget is empty; with this MR the alert appears.

MR acceptance checklist

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