Enforce allowedRoutes namespace constraints in HTTPRoute reconciliation

[lukzhang]

This PR adds logic to enforce Gateway allowedRoutes.namespaces constraints during HTTPRoute reconciliation.

• Fetches the parent Gateway
• Evaluates whether the HTTPRoute's namespace is permitted by the Gateway's allowedRoutes policy
• Updates route status with Accepted=False if the namespace is not allowed

This prevents unintended provisioning of resources from unauthorized namespaces and aligns with Gateway API spec.

[rlymbur]

I see a similar change has been made on #845 that is more exhaustive.