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/Server/Session/SessionTest.php

@@ -272,7 +272,7 @@ class SessionTest extends TestCase
         $this->assertNotEquals($session1->getId()->toRfc4122(), $session2->getId()->toRfc4122());
     }
 
-    public function testAll()
+    public function testAll(): void
     {
         $store = $this->getMockBuilder(InMemorySessionStore::class)
             ->disableOriginalConstructor()
@@ -289,7 +289,7 @@ class SessionTest extends TestCase
         $this->assertEquals(['foo' => 'bar'], $result);
     }
 
-    public function testSaveBeforeReadInitializesData()
+    public function testSaveBeforeReadInitializesData(): void
     {
         $store = new InMemorySessionStore();
         $session = new Session($store);
@@ -298,7 +298,7 @@ class SessionTest extends TestCase
         $this->assertTrue($session->save());
     }
 
-    public function testAllReturnsEmptyArrayForNullPayload()
+    public function testAllReturnsEmptyArrayForNullPayload(): void
     {
         $store = $this->getMockBuilder(InMemorySessionStore::class)
             ->disableOriginalConstructor()