Pin generated URLs to the configured domain

[melroy89]

Summary

• Pin the Symfony router context to the configured instance domain before absolute URLs are generated.
• Normalize the configured host, scheme, and ports once when the subscriber is constructed.
• Log structured warnings when a request-derived router context diverges from the configured canonical context.
• Reapply the canonical context for main requests, sub-requests, and after Symfony restores a parent request on kernel.finish_request.
• Add focused regression coverage for hostile hosts, custom ports, logging, and sub-request lifecycle resets.

Related to GHSA-v54w-hc3m-68pq.

Testing

• vendor/bin/phpunit --no-configuration --bootstrap vendor/autoload.php tests/Unit/EventSubscriber/RouterContextHostSubscriberTest.php
• PHP CS Fixer dry run for the subscriber and its unit test