setup CI to support multiple databases

[kholdrex]

This PR expands our GitHub Actions CI to run tests on both PostgreSQL and MySQL, in addition to the Ruby/ActiveRecord matrix we already use. Specifically:

1. PostgreSQL & MySQL Jobs

• We define two new jobs – test_postgres and test_mysql – each spinning up their respective database service.
• Our existing matrix for (ruby-version, activerecord-gemfile) is reused for each DB, ensuring wide coverage.

2. Local SQLite

• We omit SQLite from CI (we can test that locally) to keep the build simpler and shorter.

3. Looser Test Matching

• Updated a few specs to allow flexible quoting (backticks vs. double quotes) and minor differences in boolean or numeric representations. This ensures that tests pass on both MySQL and Postgres without failing on quoting style.