Improve wording on find_by note regarding returning only one record [ci skip]

The Engines guide used an inline namespace, `Blorgh::ApplicationController`, which caused problems for users using an
explicit module namespace because the `ApplicationController` inherited from wasn't fully qualified.
The controller qualification was fixed in 66147932.

This ditches the inline namespace and makes the example more consistent with others in the guide.

Fix #has_secure_token documentation

It's actually #validates_uniqueness_of that can generate a race condition
rather than #validates_presence_of.

Error message testing fix

The testing of error messages have been implemented wrongly a few times.
This is an attempt to fix it.

For example, some of these test should have failed with the new code.
The reason they are not failling with the new string is the fact they
were not being tested beforehand.

ActiveModel::Errors

Fixes #16518

move cache_digests rake methods into their own namespace

Don't use shorthand match on routes with inappropriate symbols

These methods are nodoc so we should not document them.

Isolate access to @associations_cache and @aggregations_cache to the Associations and Aggregations modules, respectively.

Move the `validate!` method to `ActiveModel::Validations`.

Simplify find_by_statement_cache interaction

Active Job Guide: Use sentence case for consistency in comments [ci skip]

[ci skip]

Fix cache_helper comment ERB typo

Always reset changed attributes in becomes

mail_to helper method fix

Format the time string according to the precision of the time column

Allow `method: "all"` as a valid routing test option

fix mistype in doc about \z regexp

Rename default_form_builder to avoid collision

Corrects the API to method select_tag

use `messages` instead of deprecated `ActiveModel::Errors#[]=` method [ci skip]

The 'selected' option is not doing what it should do.
The expected behavior is to pass the value selected by default for the options_from_collection_for_select method