Add throw-points test for mixed~null (non-nullable) nullsafe method call

Add a companion test case where the left operand of ?-> is mixed~null
(obtained via null-narrowing). Since the value can never be null, ?->
never short-circuits and variables assigned in arguments have Yes certainty.

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

Author: phpstan-bot

tests/PHPStan/Analyser/nsrt/throw-points/php8/null-safe-method-call.php

@@ -48,6 +48,20 @@ function () {
 	}
 };
 
+function () {
+	$result = doesntThrow();
+	if ($result === null) {
+		return;
+	}
+	// $result is mixed~null, so ?-> never short-circuits
+	try {
+		$result?->{$foo = 1}($bar = 2);
+	} finally {
+		assertVariableCertainty(TrinaryLogic::createYes(), $foo);
+		assertVariableCertainty(TrinaryLogic::createYes(), $bar);
+	}
+};
+
 function () {
 	try {
 		maybeThrows()?->{$foo = 1}($bar = 2);