Upgrade plugin to CakePHP 6

[ADmad]

Dependencies:

• cakephp/cakephp ^6.0, php >=8.4, cakephp-codesniffer ^6.0, phpunit ^13.0, rector ~2.6.0 (plus minimum-stability dev).

Source changes (cakephp60 rector set + manual follow-up):

• Entity access API renamed by Cake 6: setAccess()/accessibleFields became setPatchable()/patchableFields (Marshaller option included).
• QueryInterface methods now declare static return types; Query::order() removed, applyOptions() maps order to orderBy().
• Finder option arrays are rejected with an InvalidArgumentException pointing at named arguments instead of being deprecated.
• FixtureInterface::insert()/truncate() return void.
• Document stores the Elastica result in $searchResult, and its clean path writes values through setRawValue() so declared fields stay initialised.
• Index::saveMany() assigns the ES id/version with set() instead of direct property writes (Property PHPStan error on EntityInterface).
• ValidationRule is readonly in Cake 6: DocumentContext reads ->name/->pass.
• Plugin classes drop the underscore prefix convention (Query, Index, Connection, Marshaller, DocumentContext, IsUnique, ElasticLogger, ...); dynamic ES fields such as _version are left untouched.
• Test plugin classes added for TestPlugin/TestPluginTwo: Cake 6 removed the BasePlugin fallback in PluginCollection::create().

Tooling:

• rector.php targets PHP 8.4 and skips rector 2.5/2.6 docblock removal rules, mirroring debug_kit, so composer rector-check stays green without losing @return $this that PHPStan needs for fluent interfaces.

CI/docs:

• ci.yml pushes to 6.x, PHP matrix 8.4/8.5, prefer-lowest on 8.4.
• docs-validation uses the cakephp/.github 6.x workflow.
• deploy_docs_6x.yml added alongside the other major-version deployers.
• README, docs version references and a new 6.0 upgrade guide.