fix(model-services): handle null payloads, surface partial failures, and bypass Codex /v1/models

[asujithan]

Follow-up fixes on top of #151 UC model-services discovery path. All changes gated on UCODE_USE_MODEL_SERVICES / state["use_model_services"] unless noted.

Bug fixes

• Null-payload crash — guard data.get("model_services") or [] in list_model_services; the API returns {"model_services": null} for empty metastores instead of [], which previously crashed with TypeError.
• Silent pagination failures — propagate last_reason from list_model_services and discover_model_services even on partial success, so a 504 on page 2 surfaces during configure instead of being swallowed.
• Codex /v1/models rejection — when on the model-services path, write ~/.ucode/codex-model-catalog.json and set model_catalog_json in ~/.codex/ucode.config.toml. Codex switches to StaticModelsManager and never calls the AI Gateway's /v1/models (which currently rejects system.ai.* ids). File is removed on toggle-off and on ucode revert.
  UX
• ucode status displays Model discovery: model-services (system.ai.) vs ai-gateway (databricks-) (ungated, cosmetic).
• ucode usage — simplify_model_name strips both databricks- and system.ai. prefixes so rows merge across modes (ungated, no-op for AI Gateway data).

Tests
Unit coverage for each fix above. Full suite: 748 passed, 32 deselected (e2e + write-restricted revert). Lint + format clean.