Expose structured service tiers in app-server

[aibrahim-oai]

Summary

• Add serviceTiers: [{ id, name, description }] to app-server model/list payloads and generated schemas/SDK.
• Keep additionalSpeedTiers serialized and mark it deprecated.
• Continue mapping legacy app-server serviceTier values through the compatibility enum from the previous PR.
• Update app-server docs and model-list expectations.

Compatibility

This is additive for model/list. Existing additionalSpeedTiers and serviceTier clients continue to work; new clients should read serviceTiers for structured metadata.

Verification

• just fmt
• Schema/type fixtures updated.
• Did not run tests locally; relying on CI per project guidance.

[aibrahim-oai]

This change is part of the following stack:

• Use structured service tiers across core and app-server #20822
  • Expose structured service tiers in app-server #20823 ◀
    • Drive TUI service-tier commands from model metadata #20824

_{Change managed by git-spice.}