[Eval > Datasets] Validation message unclear when Per turn field changed to Shared with conflicting values

## EPAM AI DIAL Admin version
Admin: [FE]0.20.0-dev.72 [BE]0.20.0-dev.11

## How to reproduce
1. Login to Admin
2. Go to Evaluation > Datasets, open a dataset with a multi-turn test case that has a field with Scope = Per turn (e.g. field "sha", Turn 1 = 1, Turn 2 = 2)
3. Go to Schema tab and change that field's Scope from Per turn to Shared, save
4. Go back to Test Cases tab and open the info icon on the now-Invalid multi-turn test case

## Actual result
The validation tooltip shows the generic message "Unknown data field 'sha'" for each turn. It does not explain that the field became Shared while the turn still holds a per-turn value, and gives no hint on how to resolve it.

## Expected result
The validation message for this specific case should clearly state that field "sha" is now scoped as Shared but still has conflicting per-turn values, so the user understands why the case is Invalid and what needs to change (e.g. remove or reconcile the per-turn values).

## Confidential information
I confirm that do not share any confidential information

Start by reproducing the Evaluation > Datasets flow with a multi-turn case whose Per turn field is changed to Shared, then trace the backend validation that produces the tooltip message. Update the validation result so this conflicting-value case explains the scope mismatch and remediation, and verify that the invalid test case shows the specific message rather than the generic unknown-field message.

Written by the indexing model from the issue text.