Fix PHPStan 2.2 sealed array shape violations (#62)

PHPStan 2.2 enforces sealed array shapes more strictly, surfacing two
existing type mismatches:

- readExistingErrors() declared a return type without 'identifier', but
  delegated to decodeBaseline() which includes it. Strip the key so the
  runtime shape matches the declared type.
- SplitterTest::getSampleErrors() declared its ignoreErrors as
  array{0: ...} (sealed, single index) but returned a list of 4 entries.

Co-Authored-By: Claude Code

Author: janedbal

src/BaselineSplitter.php

@@ -164,11 +164,20 @@ private function readExistingErrors(
         }
 
         try {
-            return $handler->decodeBaseline($filePath);
+            $decoded = $handler->decodeBaseline($filePath);
 
         } catch (ErrorException $e) {
             return null;
         }
+
+        $errors = [];
+
+        foreach ($decoded as $error) {
+            unset($error['identifier']);
+            $errors[] = $error;
+        }
+
+        return $errors;
     }
 
     /**

tests/SplitterTest.php

@@ -467,7 +467,7 @@ public function testOnlyFilesWithMatchingExtensionAreDeleted(): void
     }
 
     /**
-     * @return array{parameters: array{ignoreErrors: array{0: array{message?: string, rawMessage?: string, count: int, path: string, identifier?: string}}}}
+     * @return array{parameters: array{ignoreErrors: list<array{message?: string, rawMessage?: string, count: int, path: string, identifier?: string}>}}
      */
     private function getSampleErrors(): array
     {