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/Capability/Discovery/Fixtures/SubDir/HiddenTool.php

@@ -16,7 +16,7 @@ use Mcp\Capability\Attribute\McpTool;
 class HiddenTool
 {
     #[McpTool(name: 'hidden_subdir_tool')]
-    public function run()
+    public function run(): void
     {
     }
 }