Fix duplicate escaping of quotes for check constraint expressions in MySQL schema

[Flixt]

Summary

Mysql automatically escapes quotes in generated check constraints expressions.
When Rails dumps the schema (into schema.rb) the generated schema also contains
the quotes with duplicated escaping (\\').
To fix this, we remove the escaping that MySQL provides from the fetched expression.

Fixes #42424

Other Information

I decided to go with an MySQL specific test, because putting it in https://github.com/rails/rails/blob/6-1-stable/activerecord/test/cases/migration/check_constraint_test.rb leads to code like this:

if current_adapter?(:Mysql2Adapter)
  assert_equal "`name` <> 'forbidden_string'", constraint.expression
elsif current_adapter?(:PostgreSQLAdapter)
  assert_equal "(name)::text <> 'forbidden_string'::text", constraint.expression
else
  assert_equal "name != 'forbidden_string'", constraint.expression
end

As the code shows, even MariaDB handles the quoting differently than MySQL.

[Flixt]

Is this bugfix something we'd consider worth to be backported to 6.1 / 5.2 ?

[rails-bot]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.
Thank you for your contributions.

[marcelolx]

+1

[Flixt]

I rebased it onto main..

[rails-bot]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.
Thank you for your contributions.

[HolyWalley]

+1

[Flixt]

I rebased it onto main.