Add return type annotations flagged by DebugClassLoader on Drupal 12

Problem/Motivation

DebugClassLoader reports methods that lack a native return type. For example:

Method "Drupal\Core\Form\FormInterface::buildForm()" might add "array" as a native return type declaration in the future. Do the same in implementation "Drupal\simple_oauth\Form\OpenIdConnectSettingsForm" now to avoid errors or add an explicit @return annotation to suppress this message.

On Drupal 11.4, it flags UserIdentityProvider::getUserEntityByIdentifier() and two jsonSerialize() methods. On Drupal 12, it flags 170 methods in 33 classes. TokenAuthUser has 102 of them. These deprecations fail the "phpunit (next major)" job in #3603030: Automated Drupal 12 compatibility fixes for simple_oauth 6.1.x-dev.

Steps to reproduce

Run the "phpunit (next major)" CI job.

Proposed resolution

Add accurate @return annotations to flagged methods in classes that are public API in 6.1.x. This removes the class-loader warnings without changing method signatures.

Add native return types to SimpleOauthAuthenticationProvider, OpenIdConnectSettingsForm and DisallowSimpleOauthRequests. These classes are marked @internal.

TokenAuthUser is also @internal, but it gets @return annotations. Most of its methods return the value from the decorated User. Fluent methods such as addRole() return the decorated User, not the TokenAuthUser. A native static type on those methods throws a TypeError.

Five warnings come from core traits: RedundantEditableConfigNamesTrait::getEditableConfigNames() and the CacheableDependencyTrait methods on ScopeEntity. Drupal 12 does not change these traits. Core ignores the same warnings for its own classes. Suppress these five warnings using two narrowly scoped patterns in a project .deprecation-ignore.txt. Set DRUPAL_DEPRECATION_FILTER_CONFIG in .gitlab-ci.yml so PHPUnit uses that file on Drupal 11.5 and later.

Add native return types to the other classes in the next major version.

Test-only MR !250 applies this branch on top of #3603030: Automated Drupal 12 compatibility fixes for simple_oauth 6.1.x-dev. Its "phpunit (next major)" job reports no return type deprecations from simple_oauth classes.

Remaining tasks

• Review the MR.
• Coordinate TokenAuthUser changes with #3507076: Decoupled TokenUserInterface from Oauth2TokenInterface.
• Open a follow-up for native return types in the next major version.
• Remove the .deprecation-ignore.txt entries when core adds return types to the traits.

User interface changes

None.

API changes

No public API signature changes in 6.1.x. Native return declarations are added only to internal classes. DebugClassLoader can warn subclasses that override these methods without a return type. Its vendor-prefix and version checks decide when it does.

Data model changes

None.