Mark internal classes with @internal to clarify public API boundaries

Add @internal PHPDoc tag to implementation classes that define their own constructor across domain and all submodules. Mark all hook classes as final. Add @final to DomainNegotiationContext. Add @phpstan-ignore comments to legacy hook stubs and submodule classes that reference internal classes.

Classes without their own constructor are NOT marked @internal since their constructor signature is inherited and not ours to break.

What changed:

- 55 classes marked @internal (services, event subscribers, path processors, access checkers, and other classes with custom constructors)
- 16 hook classes marked final (hook classes are implementation details, never meant to be extended)
- 1 class marked @final (DomainNegotiationContext - injectable but not extendable)
- 58 @phpstan-ignore comments added to .module/.install files for legacy hook stubs calling internal classes
- 2 @phpstan-ignore-next-line on submodule classes extending internal parents (DomainConfigUILanguageManager, DomainSourceElementManager)
- domain_config_test.install: replaced @phpstan-ignore-next-line with @var interface type hints

Public API (NOT marked @internal):

- All *Interface files
- DomainNegotiationContext (@final)
- Base classes extended by submodules: DomainBlockBase, DomainControllerBase, DomainCommands, DomainAccessControlHandler, DomainElementManager, DomainConfigLanguageManager
- Extensible Views access plugins: DomainAccessContent, DomainAccessEditor, DomainContentAccess, DomainEditorAccess
- Action base classes: DomainAccessActionBase, DomainSourceActionBase
- DomainSourceElementManager, DomainConfigMigration
- All exception classes, traits, event constants/objects
- All classes without a custom constructor (entities, forms, controllers, plugins, constraints, etc.)