Add tests for multiple callable parameter acceptors purity check

Tests the code path where a variable holds a union of closures
(multiple callable parameter acceptors): one test with mixed
pure/impure closures verifying no narrowing, and one with all
pure closures verifying narrowing still works.

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

Author: phpstan-bot

tests/PHPStan/Analyser/nsrt/bug-12686.php

@@ -21,3 +21,43 @@
 if ($h()) {
 	assertType('true', $h());
 }
+
+// Multiple callable parameter acceptors (union of closures)
+// When one variant is impure, the combined result should be impure
+/** @phpstan-impure */
+$impure = function (): bool {
+	return (bool) rand(0, 1);
+};
+
+$pure = function (): bool {
+	return true;
+};
+
+if (rand(0, 1)) {
+	$g = $impure;
+} else {
+	$g = $pure;
+}
+
+if ($g()) {
+	assertType('bool', $g());
+}
+
+// Multiple callable parameter acceptors where all are pure
+$pure1 = function (): bool {
+	return true;
+};
+
+$pure2 = function (): bool {
+	return true;
+};
+
+if (rand(0, 1)) {
+	$p = $pure1;
+} else {
+	$p = $pure2;
+}
+
+if ($p()) {
+	assertType('true', $p());
+}