Assessment: Make user prompt optional & support attachment column mapping

[vprashrex]

Issue

Closes #227

Summary

• Made the user prompt optional in the assessment flow — added an "Optional" badge and removed the prompt-required validation and step gating.
• Allowed attachment/document columns to satisfy column mapping, the topic-relevance prefilter, and submit validation (previously required at least one text column).
• Renamed provider google → google-aistudio and added anthropic to the provider type unions to match the backend.
• Replaced hardcoded "openai" provider literals with CompletionConfig["provider"].

Checklist

Before submitting a pull request, please ensure that you mark these task.

• Ran npm run dev and npm run build in the repository root and test.
• If you've fixed a bug or added code that is tested

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