The PHPStan failure is really weird:

$ phpstan a tests/Platforms/AbstractPlatformTestCase.php

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 ------ ------------------------------------------------------------------------------------
  Line   AbstractPlatformTestCase.php
 ------ ------------------------------------------------------------------------------------
  378    Parameter #1 $originalClassName of method PHPUnit\Framework\TestCase::createMock()
         expects class-string<T of Doctrine\DBAL\Platforms\AbstractPlatform>, string given.
  378    Unable to resolve the template type T in call to method
         PHPUnit\Framework\TestCase::createMock()

1. TestCase::createMock() doesn't use the T template (not even speaking of expecting it to be an AbstractPlatform).
2. If I replace the newly introduced template T with something else, the issue disappears:

   diff --git a/tests/Platforms/AbstractPlatformTestCase.php b/tests/Platforms/AbstractPlatformTestCase.php
   index f737da68a..d4ef1462c 100644
   --- a/tests/Platforms/AbstractPlatformTestCase.php
   +++ b/tests/Platforms/AbstractPlatformTestCase.php
   @@ -26,15 +26,15 @@ use function sprintf;
    use function str_repeat;
   
    /**
   - * @template T of AbstractPlatform
   + * @template Z of AbstractPlatform
     */
    abstract class AbstractPlatformTestCase extends TestCase
    {
   -    /** @var T */
   +    /** @var Z */
        protected $platform;
   
        /**
   -     * @return T
   +     * @return Z
         */
        abstract public function createPlatform(): AbstractPlatform;