Add conflict for nikic/php-parser v5+ to fix autoloader collision

rector/rector bundles its own vendored copy of nikic/php-parser v5.
When phpunit/php-code-coverage pulls in nikic/php-parser v5 directly,
the classes get loaded twice causing:

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

The InterpolatedString class only exists in php-parser v5 (it was
called Encapsed in v4). By conflicting with v5+, composer will
install php-parser v4.x which php-code-coverage also accepts and
which doesn't have the conflicting class names.

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": ">=5.0"
+    },
     "autoload": {
         "psr-4": {
             "Cake\\Upgrade\\": "src/"