Assessment: Improve column mapping options

[vprashrex]

Is your feature request related to a problem?
The assessment flow requires users to provide a prompt and map at least one text column, hindering workflows that only use attachment/document columns. Additionally, the config provider type does not match the backend, leading to type mismatches for model selection.

Describe the solution you'd like

• Make the user prompt optional in the assessment flow.
• Allow attachment/document columns for column mapping and topic-relevance prefiltering.
• Align provider types with the backend (google-aistudio, anthropic).

Original issue

Is your feature request related to a problem?
The assessment flow forces the user to write a prompt and to map at least one text column before proceeding or submitting. This blocks valid workflows that rely only on attachment/document columns, and the mandatory prompt is unnecessary for many runs. The config provider type also doesn't match the backend (google vs google-aistudio, missing anthropic), causing type mismatches when selecting Gemini/Claude models.

Describe the solution you'd like

• Make the user prompt optional throughout the assessment flow.
• Allow attachment/document columns (not only text) to satisfy column mapping and the topic-relevance prefilter step.
• Align provider types with the backend (google-aistudio, anthropic).