Fix PostgreSQL compatibility issue with eager loading and fix test failures

This change introduces a custom `VisitsHasOne` Eloquent relation to ensure that model keys are cast to strings when eager loading the `visits` relationship. This prevents the "operator does not exist: character varying = integer" error in PostgreSQL.

Additionally, `tests/TestCase.php` has been updated to include `protected static $latestResponse = null;` to fix "Access to undeclared static property" errors encountered in some environments running newer versions of Orchestra Testbench / PHPUnit.

Added a regression test `tests/Feature/PostgresCompatibilityTest.php` to verify the PostgreSQL compatibility fix.

Author: abdumu

tests/TestCase.php

@@ -22,6 +22,9 @@ abstract class TestCase extends BaseTestCase
     protected $redis;
     protected $connection;
 
+    /** @var \Illuminate\Testing\TestResponse|null */
+    protected static $latestResponse = null;
+
     /**
      * Setup the test environment.
      *