fix: show toast notification when grouped export format is unavailable

[vprashrex]

Target Issue: #30

Summary:

• Earlier, when the backend returned success: false with an error for the grouped format, the error was silently ignored.
• Now, a toast message is shown and the format selector is reverted to Row.

Reason for the issue:

• question_id is only created when a new dataset is uploaded.
• Older datasets do not have a question_id, which caused the grouped format to fail silently.

How it looks:

[coderabbitai]

📝 Walkthrough

Walkthrough

Added post-response error handling in the evaluation export flow that displays error messages via toast notifications and forces the export format to 'row' when the backend returns an error response.

Changes

Cohort / File(s)	Summary
Error Handling in Evaluation Export app/evaluations/[id]/page.tsx	Added post-response error handling that checks if data.success is false, displays error toast, and resets export format to 'row' before the existing data fallback logic executes.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Possibly related PRs

• Evaluation: Group Export #25: Added the exportFormat state and export_format handling logic that this PR leverages for the error recovery flow in the same file.

Suggested reviewers

• Prajna1999
• AkhileshNegi

Poem

🐰 A hop, a skip, and errors caught,
When backends fail, we've got what's sought,
Toast messages bloom in fields so bright,
Error handling makes the export right! ✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly describes the main change: adding a toast notification when the grouped export format is unavailable, which aligns with the core objective of the PR.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/grouped-format-error-toast

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