[Eval > Datasets] CSV import into schema-less dataset loses Shared field scope for multi-turn cases

## EPAM AI DIAL Admin version
Admin: [FE]0.20.0-dev.73 [BE]0.20.0-dev.12

## How to reproduce
1. Login to Admin
2. Open a dataset that has at least one Shared field (e.g. "tags" = Shared, "message"/"expected" = Per turn) and multi-turn test cases with distinct per-turn values
3. Export the dataset as CSV
4. Create a new, empty dataset (Schema has no fields)
5. Import the exported CSV into the new dataset (Import mode = Override, On name collision = Fail)
6. Confirm the import
7. Open the Schema tab of the new dataset

## Actual result
All fields — including those originally declared Shared — are classified as Per turn. Attempting to manually change any of these fields back to Shared causes all multi-turn test cases to become Invalid with "Unknown data field" errors on each turn row.

## Expected result
Fields that had identical values across all turns of a test case should be imported as Shared, or at minimum allow the user to correct the scope to Shared post-import without invalidating the test cases.

## Additional information
Known CSV format limitation — column-level scope cannot be expressed in CSV rows. Accepted as a trade-off in the fix for #120 (PR #135) per developer suggestion.

## Confidential information
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