Update PHPStan config for 2.x compatibility

PHPStan 2.x changed how @mixin handles static return types: instead of
resolving static to the calling class, it resolves to the mixin type
itself (Mixin interface). This causes chain navigation methods (all(),
key(), property()) called after assertion methods to appear undefined on
the Mixin type, cascading into method.nonObject errors.

Remove the phpt file extension (no .phpt files remain after the switch
to PHPUnit integration tests). Retain the existing regex for undefined
Mixin method calls and add a new identifier-scoped rule to suppress the
cascading method.nonObject false positives in integration tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: henriquemoody

phpstan.neon.dist

@@ -3,8 +3,8 @@ parameters:
   paths:
     - src/
     - tests/
-  fileExtensions:
-    - php
-    - phpt
   ignoreErrors:
     - '/Call to an undefined method Respect\\Assertion\\Mixin\\Chain\\Mixin::[a-zA-Z]+\(\)/'
+    - identifier: method.nonObject
+      paths:
+        - tests/integration/