[sachalmalick]

Summary

Closes #17275 (discovery metadata prerequisite).

Global Ingress polls /pools, but the MCS backend previously returned no model or frontend addresses. Add optional --routing-config model/location bindings and join them with ready imported frontend EndpointSlices by source cluster, namespace, and DGD name. Frontend addresses remain dynamic; a ready Relay is still required for pool membership. Existing constructors and unconfigured behavior remain compatible.

This is the first PR in a stack; the deployment and validation example follows separately.

Validation

• 75 targeted tests passed across global-discovery-client, global-discovery-service, global-router, and global-ingress.
• Added coverage for frontend replacement/readiness, Relay withdrawal, identity isolation, IPv6 URLs, and invalid configuration.
• Targeted Rust formatting, diff whitespace, and strict changed-file CODEOWNERS checks passed.

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