style: apply php-cs-fixer and PHPStan fixes (#289)

* style: apply php-cs-fixer to test files

Addresses drift flagged by the default PHP-CS-Fixer config — mostly
adds missing `: void` return types on pre-existing test methods.

No behavioural changes.

* fix: resolve phpstan errors surfaced by qa

- Drop the obsolete `Method ::test*() has no return type specified`
  ignore rule — every test method now declares a return type, so
  the pattern never matches and phpstan reports it as unmatched.
- Remove `?? []` from two reads of `CallToolRequest::$arguments`;
  the property is typed `array` (non-nullable), so the coalesce is
  a no-op that phpstan flags.

---------

Co-authored-by: DDEV User <nobody@example.com>

tests/Unit/Schema/Request/CreateSamplingMessageRequestTest.php

@@ -20,7 +20,7 @@ use PHPUnit\Framework\TestCase;
 
 final class CreateSamplingMessageRequestTest extends TestCase
 {
-    public function testConstructorWithValidSetOfMessages()
+    public function testConstructorWithValidSetOfMessages(): void
     {
         $messages = [
             new SamplingMessage(Role::User, new TextContent('My name is George.')),
@@ -34,7 +34,7 @@ final class CreateSamplingMessageRequestTest extends TestCase
         $this->assertSame(150, $request->maxTokens);
     }
 
-    public function testConstructorWithInvalidSetOfMessages()
+    public function testConstructorWithInvalidSetOfMessages(): void
     {
         $this->expectException(InvalidArgumentException::class);
         $this->expectExceptionMessage('Messages must be instance of SamplingMessage.');