Remove shipmonk identifier requirement from RuleTestCase

Make the test case more generic by removing the hardcoded requirement
that error identifiers must start with 'shipmonk.'. This allows the
utility to be used by any project with their own identifier naming.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: JanTvrdik

README.md

@@ -92,7 +92,6 @@ public function testRule(): void
 
 Every error is automatically validated:
 - ✅ Must have an identifier
-- ✅ Identifier must start with `'shipmonk.'`
 - ✅ Errors are matched to specific line numbers
 
 ## Advanced Usage

src/RuleTestCase.php

@@ -58,7 +58,6 @@ protected function processActualErrors(array $actualErrors): array
             $resultToAssert[$key] = $this->formatErrorForAssert($error->getMessage(), $usedLine);
 
             self::assertNotNull($error->getIdentifier(), "Missing error identifier for error: {$error->getMessage()}");
-            self::assertStringStartsWith('shipmonk.', $error->getIdentifier(), "Unexpected error identifier for: {$error->getMessage()}");
         }
 
         ksort($resultToAssert);