test(combos): preserve configured routing prefix in legacy builder contract

[diegosouzapw]

Summary

Align the older builder-route contract with #14143 / #14135 (f4d7d8a, Rafa Martins): custom provider model options route through the configured public prefix, while their providerId remains the internal node ID. Select the custom model by ID rather than relying on array position.

No production changes, loosened assertions or timeout changes.

Evidence

• Exact base 373c31f: two PASS and one assertion failure, gd/gpt-custom versus stale internal-node prefix.
• Updated route suite plus original fix(api): /v1/models catalog emits a degraded duplicate entry keyed by the node UUID (vision dropped, context_length 128000) and input_modalities=null on the aliased entry #14135 alias/serialization regressions: four PASS, zero failures/skips.
• Prettier, ESLint with existing frozen suppressions, diff check and complete branch hooks passed.

Draft recovery slice. Combined CI and protected-branch rollout remain pending.

[diegosouzapw]

Fechando como coberta pela #14511, mergeada na release/v3.8.51 (drenou as mesmas base-reds: genérico de stdio do auggie, imports do projectCombo, strip de _native*Passthrough e os guards de catálogo/prefixo/qwen/opencode/quota). O conteúdo que só esta PR tinha já entrou pelas #14501, #14503, #14508 e #14516. (merge-batch 2026-09-24)