Redefine isolation level method as MSSSQL resets isolation after transaction

[aidanharan]

Redefine isolation level method as MSSSQL resets isolation level after transaction unlike MySQL/PostgreSQL.

https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/actions/runs/14250370905/job/39941435835

 1) Failure:
TransactionIsolationTest#test_0003_default_isolation_level [/usr/local/bundle/bundler/gems/rails-e0452e80137c/activerecord/test/cases/transaction_isolation_test.rb:80]:
Expected: 1
  Actual: 2