Migrate Kusto test utilities and integration helpers off legacy kusto client

## Context
Several test utilities and integration helpers still use the legacy root `github.com/Azure/azure-kusto-go/kusto` client. These are not the main production dependency drivers, but they will keep legacy imports around after production paths migrate.

## Files
- `pkg/testutils/kql_verify.go`
- `pkg/testutils/integration_test.go`
- `pkg/testutils/kustainer/kustainer.go`

## Current legacy usage
- Creates Kusto clients against kustainer/local ADX with `kusto.NewConnectionStringBuilder` and `kusto.New`.
- Executes management commands and queries with `client.Mgmt` / `client.Query`.
- Builds statements with legacy `kusto/kql`.
- Reads results with `NextRowOrError` and `ToStruct`.

## Helper behaviors to preserve
- Verify table existence.
- Verify function existence and retrieve function metadata.
- Verify table row counts.
- Verify table schemas via `getschema`.
- Create volatile databases in kustainer.
- Alter ingestion batching policy.
- Query integration-test health metrics.

## Acceptance criteria
- Test utility packages no longer import `github.com/Azure/azure-kusto-go/kusto...` packages.
- Existing integration helper behavior remains available for tests.
- Kustainer/local endpoint support remains working.

Start with pkg/testutils/kql_verify.go, pkg/testutils/integration_test.go, and pkg/testutils/kustainer/kustainer.go, then trace the replacement Kusto client used by migrated production paths. Run the integration tests and verify that legacy imports are gone while table, function, schema, row-count, kustainer, batching-policy, and health-metric helpers still work.

Written by the indexing model from the issue text.