Add conflict for nikic/php-parser to fix autoloader collision

rector/rector bundles its own vendored copy of nikic/php-parser.
When phpunit/php-code-coverage (a transitive dependency) pulls in
nikic/php-parser directly, the classes get loaded twice causing:

  Cannot declare class PhpParser\Node\Scalar\InterpolatedString,
  because the name is already in use

Adding this conflict prevents composer from installing nikic/php-parser
directly, avoiding the autoloader collision with rector's bundled version.

Fixes CI failure: https://github.com/cakephp/upgrade/actions/runs/19643729518

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: dereuromark

composer.json

@@ -11,6 +11,9 @@
         "rector/rector": "~2.2.3",
         "symfony/string": "^6.0 || ^7.0"
     },
+    "conflict": {
+        "nikic/php-parser": "*"
+    },
     "autoload": {
         "psr-4": {
             "Cake\\Upgrade\\": "src/"