Remove legacy Kusto HttpError parsing after old client paths migrate

## Context
`pkg/kustoutil.ParseError` currently supports both new `azkustodata/errors.HttpError` and legacy `kusto/data/errors.HttpError`.

The legacy branch is compatibility glue for remaining old Kusto client call sites.

## Files
- `pkg/kustoutil/errors.go`
- `pkg/kustoutil/errors_test.go`

## Current legacy usage
- Imports `github.com/Azure/azure-kusto-go/kusto/data/errors` as `legacykustoerrors`.
- Detects `*legacykustoerrors.HttpError` with `errors.As`.
- Calls `UnmarshalREST()` and extracts `error["@message"]`.

## Migration considerations
This should likely be one of the final cleanup issues, after production code and tests no longer produce legacy Kusto errors.

## Acceptance criteria
- `pkg/kustoutil` no longer imports legacy `github.com/Azure/azure-kusto-go/kusto/data/errors`.
- `ParseError` still handles `azkustodata/errors.HttpError` correctly.
- Tests cover supported error formats without relying on legacy Kusto types.

Read pkg/kustoutil/errors.go and pkg/kustoutil/errors_test.go, then check production call sites and tests to confirm they no longer produce legacy Kusto errors. Remove the legacy dependency only after that migration is complete, while preserving parsing for azkustodata/errors.HttpError. Done means the package has no legacy import and tests cover supported formats without legacy types.

Written by the indexing model from the issue text.