feat(codex): reroute GET /models to OpenAI-shaped discovery for IntelliJ

[IceRhymers]

What

OpenAI-shaped clients — notably JetBrains IntelliJ's AI Assistant, pointed at the local proxy root — enumerate models via GET /models. The codex proxy forwards to the {host}/ai-gateway/openai/v1 base path, but Databricks has no /ai-gateway/openai/v1/models endpoint — a naive forward 400s with "doesn't match any known API type". The working discovery endpoint is {host}/ai-gateway/v1/models (no /openai/v1 segment), but it returns a Databricks-shaped list (query_name/display_name), not OpenAI's {id, object:"model"}.

This adds an opt-in GET /models reroute to the shared proxy engine, gated on a new Config.OpenAIModelsUpstream field.

How

• internal/core/proxy/models_reroute.go — when OpenAIModelsUpstream is set, register GET /models (exact-match), fetch the discovery URL with a fresh token, and transform the body to the OpenAI shape (id=query_name, object:"model", owned_by:"databricks"). Non-GET /models and /models/{id} fall through to the inference catch-all.
• Thread the field through LaunchPlan → core.Run → proxy.Config, and through the codex ProxyConfig facade (serve_codex.go bypasses core.Run).
• New ConstructModelsURL(host) in codex token.go (const /ai-gateway/v1/models), single-sourced into both launch_codex.go (wrapper) and serve_codex.go (serve).

Off by default

Claude and opencode leave OpenAIModelsUpstream empty, so the route is never registered — byte-identical to before. The multiplexer (databricks-agents codex serve) is covered automatically via exec-delegation.

Testing

• 5 new shared-engine tests + a codex ConstructModelsURL test; full affected suites green.
• Live against fevm-west: both databricks-codex serve and databricks-agents codex serve return 44 OpenAI-shaped models on GET /models; POST /models still forwards to inference.

Note

The transformed created field is a fixed constant — the gateway exposes no per-model creation time. OpenAI clients only require the field present and numeric; revisit if a client sorts by it.

This pull request and its description were written by Isaac.