Add rector, and split lint off test

[DerManoMann]

Two commits, both of which were sitting uncommitted in a working tree.

Split the lint step off test. composer test ran phpunit and then @lint. No CI job
uses it that way — code-style runs composer lint, static-analysis runs composer analyse,
and the adapter jobs call vendor/bin/phpunit directly — so the coupling only slowed down a
local test run. test is phpunit alone now.

Add rector. rector.php uses PhpVersion::PHP_81 (matching "php": ">=8.1" and the
8.1–8.4 CI matrix), the prepared sets, and TypedPropertyFromStrictConstructorRector.
composer rector applies it; composer analyse ends with @rector --dry-run.

Why the dry run lives with analyse and not with lint

laravel/laravel and slim/slim are composer suggests, so nothing installs them by default.
static-analysis.yml already does composer require for both before composer analyse, because
phpstan is meaningless on the adapters without them. code-style.yml, where lint runs, installs
neither.

Run without Laravel, rector reads tests/Adapters/LaravelAdapterTest against a base class it
cannot resolve and proposes two changes that are both wrong:

+    public $app;                 // CompleteDynamicPropertiesRector
-        parent::setUp();         // RemoveParentCallWithoutParentRector

$app comes from Illuminate\Foundation\Testing\TestCase and dropping parent::setUp() would
skip Laravel's application bootstrap. Putting the check in the job that installs the frameworks
is what stops that being proposed on every run.

What the first rector run produced, and the one thing corrected

Kept: typed properties on PSR17Middleware::$psrHttpFactory and
Slim\OpenApiVerifierMiddleware::$container, nested if/else → elseif, '/' != $path[0] →
!==, [0-9]+ → \d+, if ($path) → if ($path !== []), and setUp() visibility on the
Laravel adapter test.

Corrected: typing the loader property in VerifiesOpenApi also dropped its = null, leaving a
typed property uninitialised. getOpenApiSpecificationLoader() opens by reading it, so the first
call fatals with Typed property … must not be accessed before initialization. The suite stays
green either way — VerifiesOpenApiTest overrides that accessor, and the code that does reach it
(Adapters\AbstractOpenApiResponseVerifier) is only exercised by the adapter tests, which skip
unless the frameworks are installed. Restored to = null.

Verified

vendor/bin/phpunit — 28 tests, 8 skipped (Laravel and Slim absent locally), green.
php-cs-fixer --dry-run — clean. rector --dry-run — clean apart from the two
framework-absent proposals above, which is what CI's framework-installing job is there to settle.