fix: accept underscores in domain hostnames for API URL validation

PHP's FILTER_VALIDATE_URL rejects underscores in the host, so domains
like https://myapp_service.example.com were rejected by the API and
never got a Let's Encrypt certificate. Add an isValidDomainUrl() helper
that validates a copy with underscores replaced by hyphens, and route
domain validation in the Applications and Services API controllers
through it.

Fixes #10597

tests/Unit/IsValidDomainUrlTest.php

@@ -0,0 +1,22 @@
+<?php
+
+it('accepts hostnames containing underscores', function () {
+    // Regression: PHP's FILTER_VALIDATE_URL rejects underscores in the host,
+    // which blocked valid service domains (e.g. Docker service naming) from
+    // being saved and getting Let's Encrypt certificates. See issue #10597.
+    expect(isValidDomainUrl('https://myapp_service.example.com'))->toBeTrue();
+    expect(isValidDomainUrl('http://my_app.example.com'))->toBeTrue();
+    expect(isValidDomainUrl('https://a_b_c.example.com/path'))->toBeTrue();
+});
+
+it('accepts ordinary domains and URLs', function () {
+    expect(isValidDomainUrl('https://example.com'))->toBeTrue();
+    expect(isValidDomainUrl('http://sub.example.com:8080/path?q=1'))->toBeTrue();
+    expect(isValidDomainUrl('https://example.com/a_b'))->toBeTrue();
+});
+
+it('rejects strings that are not valid URLs', function () {
+    expect(isValidDomainUrl('not a url'))->toBeFalse();
+    expect(isValidDomainUrl('example.com'))->toBeFalse();
+    expect(isValidDomainUrl(''))->toBeFalse();
+});