Fix up SQLServer to return numeric array.

[dereuromark]

Resolves #952

Issue: cakephp/phinx#2386
Impact: High - BC breaking bug
Status in Migrations: ✅ CONFIRMED BUG EXISTS
Location: src/Db/Adapter/SqlserverAdapter.php:316

The SqlServerAdapter::getColumns function returns an associative array of columns, where the keys are column names. All other adapters return a numeric array which is also expressed by our AdapterInterface

// Current (WRONG):
$columns[$columnInfo['name']] = $column;

// Should be:
$columns[] = $column;

The AdapterInterface specifies Column[] (numeric array) but SqlServer returns an associative array keyed by column names. This violates the contract and affects any code iterating over columns.