Correct DomainTenantResolver::isSubdomain() check (#1425)

Added a failing test for determining if a host is a subdomain, then
fixed `DomainTenantResolver::isSubdomain()` (similar fix as in #1423)
and a related assertion.

Previously, while having `tenancy.identification.central_domains` set to
e.g. `['site.com']`, the `isSubdomain()` check consider `tenantsite.com`
a subdomain because it ends with `site.com`. Now, instead of the
`endsWith()` check, the method checks if the passed domain is in the
configured central domains. If it is, it returns `false`. Otherwise,
loop through all the central domains and check if the passed domain
matches any of the central domains prefixed with a dot (e.g.
`tenant.site.com` would be considered a subdomain, `tenant.site.com`
wouldn't).

Because in InitializeTenancyByDomainOrSubdomain, if tenancy fails to
initialize using a subdomain (before this PR's changes, e.g.
`tenantsite.com` would be considered a subdomain, and `tenantsite` would
be used for initializing tenancy), it'll catch the exception and use the
whole domain for identification instead, this error will likely never be
noticed in real-world usage. So this PR corrects the subdomain detection
logic, but the real-world impact of that is negligible.

> Note: The subdomain error catching logic in domainOrSubdomain ID MW
was added in v4. If we applied this change in v3, it'd fix a real issue
where domainOrSubdomain ID MW would just fail at the subdomain
initialization, without attempting domain initialization after the
failure.

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Samuel Štancl <samuel@archte.ch>

tests/EarlyIdentificationTest.php

@@ -300,7 +300,7 @@ test('using different default route modes works with global domain identificatio
         $exception = match ($middleware) {
             InitializeTenancyByDomain::class => TenantCouldNotBeIdentifiedOnDomainException::class,
             InitializeTenancyBySubdomain::class => NotASubdomainException::class,
-            InitializeTenancyByDomainOrSubdomain::class => NotASubdomainException::class,
+            InitializeTenancyByDomainOrSubdomain::class => TenantCouldNotBeIdentifiedOnDomainException::class,
         };
 
         expect(fn () => $this->withoutExceptionHandling()->get('http://localhost/central-route'))->toThrow($exception);