When Core::call() received a 302 STATUS_FOUND response, it extracted only scheme://host
from the Location header and recursively called itself with the new domain.

On self-hosted portals with an expired license, Bitrix24 redirects every request to
/bitrix/coupon_activation.php — same host, different path. The extracted domain was identical
to the old domain, changeDomainUrl() silently accepted it, and the method called itself forever,
producing a PHP fatal stack overflow.

Fix:
After computing portalNewDomainUrlHost, compare it to portalOldDomainUrlHost.
If they are equal this is NOT a legitimate domain-migration — throw PortalUnavailableException
with the full Location URL in the message so callers and logs have the context they need.

New exception: Bitrix24\SDK\Core\Exceptions\PortalUnavailableException extends BaseException

Usage (caller side):

try {
    $response = $core->call('app.info');
} catch (PortalUnavailableException $e) {
    // Portal is unavailable (expired license, maintenance, etc.)
    // $e->getMessage() contains the redirect location for debugging
}

Test plan

• make lint-cs-fixer — passed
• make lint-rector — passed (auto-fix: unused catch variable removed)
• make lint-phpstan — passed
• make lint-deptrac — passed (0 violations)
• make test-unit — passed (663 tests, 1138 assertions)

Closes #372

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