2.x: Fix negation types for Inspector

[Niklan]

As discussed in Slack: https://drupal.slack.com/archives/C033S2JUMLJ/p1744383402099899?thread_ts=1744246451.941499&cid=C033S2JUMLJ, the current Inspector implementation doesn't handle negation at all.

For example:

$array = mixed_function();
assertType('mixed', $array);
Inspector::assertAll(fn (array $i): bool => TRUE, $array);
assertType('iterable', $array);

After passing Inspector::assertAll(fn (array $i): bool => TRUE, $array); line, PHPStan would think it is always iterable, even if it is not passed ::assertAll() and the result was FALSE. This works fine with assert(), but breaks under other conditions, for example:

$array = mixed_function();
assertType('mixed', $array);
if (!Inspector::assertAll(fn (array $i): bool => TRUE, $array)) {
    assertType('mixed~iterable', $array);
}

This leads to a test failure:

--- Expected
+++ Actual
@@ @@
-'mixed~iterable'
+'iterable'

PHPStan thinks it is iterable, but if it's not passed the assertion, it is anything else but not iterable (mixed~iterable).

It looks like PHPStan is using the ~ symbol for type negation, like mixed~string, which reads as: This type is anything (mixed), but definitely not string.

This PR fixes the issue, and previous tests pass properly as before. This PR is required for #858 because it is not possible to provide proper test cases for it unless negation is supported.