Clarify UI logic for showing table preview and upsert options in index tables

[adameat]

Based on user feedback (see screenshot), there is confusion about why the UI sometimes displays a table preview (such as for a vector index), but not for secondary indexes. Additionally, in some cases the UI suggests the 'upsert' operation where it may not be appropriate.

Please clarify:

• What determines whether the table preview is shown in the UI for different index types?
• Why is there a difference in preview availability between vector and secondary indexes?
• Should the UI be suggesting 'upsert' for index tables, or should this option be excluded?

Improving this logic would help users understand their data model better and avoid inappropriate operations.