Fix fragile test (`AssociationProxyTest#test_save_on_parent_saves_children`)

Remove implicit coercion deprecation of durations

Add documentation about signature_key for MessageEncryptor.new [ci skip]

If we run only following tests:

- test/cases/scoping/default_scoping_test.rb
- test/cases/associations_test.rb

```
$ cat Rakefile.test
require "rake/testtask"

ENV["ARCONN"] = "postgresql"

Rake::TestTask.new do |t|
  t.libs << "test"
  t.test_files = %w(
    test/cases/scoping/default_scoping_test.rb
    test/cases/associations_test.rb
  )
end
```

a test will fail:

```
$ bundle exec rake test -f Rakefile.test
/app/activesupport/lib/active_support/core_ext/enumerable.rb:20: warning: method redefined; discarding old sum
Using postgresql
Run options: --seed 11830

# Running:

.........................................................................................F................

Finished in 6.939055s, 15.2759 runs/s, 27.9577 assertions/s.

  1) Failure:
AssociationProxyTest#test_save_on_parent_saves_children [/app/activerecord/test/cases/associations_test.rb:185]:
Expected: 1
  Actual: 2

106 runs, 194 assertions, 1 failures, 0 errors, 0 skips
rake aborted!
Command failed with status (1)
/usr/local/bin/bundle:22:in `load'
/usr/local/bin/bundle:22:in `<main>'
Tasks: TOP => test
(See full trace by running task with --trace)
```

In #28083, change `self.use_transactional_tests` to `false`
but we forget to clean-up fixture.
However we don't have to disable transaction except a few tests.

In #28204 we deprecated implicit conversion of durations to a
numeric which represented the number of seconds in the duration
because of unwanted side effects with calculations on durations
and dates. This unfortunately had the side effect of forcing a
explicit cast when configuring third-party libraries like
expiration in Redis, e.g:

    redis.expire("foo", 5.minutes)

To work around this we've removed the deprecation and added a
private class that wraps the numeric and can perform calculation
involving durations and ensure that they remain a duration
irrespective of the order of operations.

Remove unused `@txn` variable

Make destroy command work within engines

Rename test of `app:update` to more appropriate name

This was added in c24c8852, removed in
b89ffe7f, and then (unintentionally?)
reintroduced in 2d7ae1b0.

`rails:update` was changed to `app:update` in 6fb31638.
Therefore, I think that it is better that the test name is also `app_update`.

Adding durations to `Time` instances is perfectly okay.

Add CHANGELOG entry for --skip-coffee [ci skip]

Was added in #25248 by @bluesh55

Remove obsolete warning about regular expression

This warning became obsolete when the regular expression was changed to
use `\z` instead of `\Z` in fce9c4e5.

    "-1234\n" =~ /\A[+-]?\d+\Z/ => 0
    "-1234\n" =~ /\A[+-]?\d+\z/ => nil

[ci skip]

This reverts commit 192db644, reversing
changes made to 98939553.

We can't sacrifice correctness for performance.

Instead of calling methods of Rails.application directly, we need to
use a method that is considered for the rails engine.

Add CHANGELOG for #26226 [ci skip]

500x memory reduction of 10k schemas for postgresql adapter

Remove non-exists method delegation

Extract `data_source_sql` to refactor data source statements

[ci skip]

[ci skip]

[ci skip]

Reads easier, and removes extra tick marks.

[ci skip]

Fix Typo [ci skip]

Arcticle --> Article

[ci skip]

[docs] fix ActionController docs

Restore action_cable.js UMD module support. Fixes #28366

Dont always display inline screenshots in system testing (#28133)

Suppress deprecation warning `implementing to_yaml is deprecated`

Fixes ActionController::Rendering#with_defaults

Simply forward `Calculations#count` to `Enumerable#count`

[ci skip]

Simply delegate `as_json` to `records`