fix: let Action Cable's PostgreSQL subscription adapter run on aws_po…

[aaron-congo]

Summary

Fix Action Cable's PostgreSQL subscription adapter (adapter: postgresql in cable.yml) refusing to run on the aws_postgresql ActiveRecord adapter.

Description

Stacked on #196 (which is stacked on #195) - review and merge those first; this PR's diff is the single commit on top of them.

Action Cable's PostgreSQL subscription adapter checks that the raw connection it gets from ActiveRecord is a PG::Connection, and raises otherwise. aws_postgresql hands out a WrapperPgConnection, so every subscribe and broadcast failed with The Active Record database must be PostgreSQL in order to use the PostgreSQL Action Cable storage adapter. Turbo Stream broadcasts (broadcasts_to, broadcast_replace_to) go through the same path and failed too. The stock postgresql adapter works.

Changes:

• New ActiveRecord::ConnectionAdapters::AwsActionCablePostgreSQLSupport, loaded by the aws_postgresql adapter. It registers an ActiveSupport.on_load(:action_cable) hook that loads Action Cable's PostgreSQL subscription adapter and prepends a verify! override. The override accepts a WrapperPgConnection and passes every other connection to Action Cable's own check.
• The calls Action Cable makes on the connection (exec, escape_identifier, escape_string, wait_for_notify) are already forwarded to the wrapped PG::Connection.
• WrapperPgConnection#is_a? is unchanged, so code that needs a real PG::Connection is not misled.
• Applications without Action Cable never run the hook. The hook works whether Action Cable loads before or after the adapter.
• Action Cable 7.2, 8.0 and 8.1 have the same check and load hook, so all supported versions are covered.

Testing:

• New spec/unit/aws_action_cable_postgresql_support_spec.rb, using a stand-in for Action Cable's adapter in spec/fixtures/action_cable_stub (Action Cable is not a dependency of the gem). It fails without the fix and checks that non-PostgreSQL connections are still rejected.
• bundle exec rspec spec/unit: 3138 examples, 0 failures. The new spec also passes under ActiveRecord 7.2 and 8.0. bundle exec rubocop lib spec: no offenses.
• Manual test against Aurora PostgreSQL with Action Cable 8.1's real PostgreSQL subscription adapter on aws_postgresql: a subscribe followed by a broadcast delivers the message, with Action Cable loaded both before and after the adapter. Before the fix the same test failed with the error above; the stock postgresql adapter passed.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.