Evals: Migrate prompt iteration to async job polling

[Ayush8923]

Is your feature request related to a problem?
The recent change to the improve-prompt endpoint has disrupted the synchronous handling of prompt iterations. Clients now receive a job_id instead of the expected configuration details, leading to failures in the iteration process.

Describe the solution you'd like
Make the following enhancements:

1. Create a new BFF proxy route at app/api/evaluations/[id]/improve-prompt/[jobId]/route.ts (GET) to poll the backend.
2. Update the existing POST proxy at app/api/evaluations/[id]/improve-prompt/route.ts to pass through the 202 response with job_id.
3. Revise handleIteratePrompt in app/(main)/evaluations/[id]/page.tsx to:
   • POST and read job_id, then initiate polling on a 2s interval until the status is no longer PENDING/PROCESSING.
   • On SUCCESS, retrieve config_id and version, clear cache, display success toast, and redirect appropriately.
   • On FAILED, show error_message in a toast.
   • On timeout, show error toast and clear loading state.

Original issue

Describe the current behavior

Context

Backend converted improve-prompt from a synchronous call to an async Celery job.

• Backend issue: Prompt Iteration: Asynchronous Anthropic call kaapi-backend#1018
• Backend PR: feat(Prompt Iteration): Move processing to Celery kaapi-backend#1023

The endpoint no longer returns the new config directly. It now returns a job_id that the client must poll until the job completes.

Current (broken) behavior

handleIteratePrompt in app/(main)/evaluations/[id]/page.tsx POSTs to /api/evaluations/{id}/improve-prompt and expects a synchronous body { success, data: { config_id, id, version } }, then redirects to the prompt editor.

With the new backend this response is 202 { job_id, status, message, ... } — no config, no success flag — so prompt iteration no longer works.

Describe the enhancement you'd like

1. New BFF proxy route app/api/evaluations/[id]/improve-prompt/[jobId]/route.ts
   (GET) forwarding to the backend poll endpoint via apiClient.
2. Update POST proxy app/api/evaluations/[id]/improve-prompt/route.ts — pass
   through the 202 + job_id body unchanged (drop the sync success/data shape
   assumption).
3. Rewrite handleIteratePrompt (app/(main)/evaluations/[id]/page.tsx):
   • POST → read job_id.
   • Poll GET on an interval (e.g. 2s, with max attempts / timeout) while status is PENDING/PROCESSING. Keep isImprovingPrompt loading state active during polling.
   • On SUCCESS: pull config_id + version from config_version (confirm ConfigVersionPublic field names), invalidateConfigCache(), success toast, redirect to /configurations/prompt-editor config=...&version=...&from=evaluations.
   • On FAILED: show error_message in an error toast.
   • On timeout: error toast, clear loading state.

[github-actions]

🎉 This issue has been resolved in version 0.4.0-main.2 🎉

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