Move AbstractAdapter#valid_type? to a class method

[aidanharan]

Refactor native database types for rails/rails#54875

Fixes (https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/actions/runs/14670746703/job/41176483142?pr=1325):

  1) Error:
ActiveRecord::AdapterTest#test_invalid_column:
NameError: undefined local variable or method 'native_database_types' for class ActiveRecord::ConnectionAdapters::SQLServerAdapter
    /usr/local/bundle/bundler/gems/rails-99e27fa586af/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb:894:in 'ActiveRecord::ConnectionAdapters::AbstractAdapter.valid_type?'
    /usr/local/bundle/bundler/gems/rails-99e27fa586af/activerecord/test/cases/adapter_test.rb:57:in 'ActiveRecord::AdapterTest#test_invalid_column'

  2) Error:
ActiveRecord::AdapterTest#test_valid_column:
NameError: undefined local variable or method 'native_database_types' for class ActiveRecord::ConnectionAdapters::SQLServerAdapter
    /usr/local/bundle/bundler/gems/rails-99e27fa586af/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb:894:in 'ActiveRecord::ConnectionAdapters::AbstractAdapter.valid_type?'
    /usr/local/bundle/bundler/gems/rails-99e27fa586af/activerecord/test/cases/adapter_test.rb:51:in 'block in ActiveRecord::AdapterTest#test_valid_column'
    /usr/local/bundle/bundler/gems/rails-99e27fa586af/activerecord/test/cases/adapter_test.rb:49:in 'Hash#each_key'
    /usr/local/bundle/bundler/gems/rails-99e27fa586af/activerecord/test/cases/adapter_test.rb:49:in 'ActiveRecord::AdapterTest#test_valid_column'