[Feature] Listing-only model exclusion (hide from /v1/models while keeping models routable)

[sofianbll]

Problem

Model exclusions remove a model from the listing and from routing. Verified on eceasy/cli-proxy-api:v7.3.6 with:

oauth-excluded-models:
  codex:
    - "*"

• GET /v1/models no longer lists the codex models (intended) ✅
• but the models also stop being routable: POST /v1/responses {"model":"codex-go/gpt-5.6-sol", ...} fails with unknown provider for model codex-go/gpt-5.6-sol ❌

Same behaviour with the per-credential form (codex-api-key[].excluded-models).

Why a listing-only variant is needed

A gateway that builds its catalogue from discovery cannot afford that trade-off. Concretely: Bifrost's model catalogue is union(upstream /v1/models, its own datasheet), so hiding another channel's models from the catalogue currently requires excluding them (breaking the calls) or disabling list_models for the provider (which also removes the models from the gateway's own /v1/models). There is no way to keep a clean catalogue and keep every model callable by clients that hardcode model names.

Proposal

Any of these would work:

1. A listing-only flag on an exclusion entry:

oauth-excluded-models:
  codex:
    - model: "*"
      listing-only: true

2. A separate option with the same shape as oauth-excluded-models, e.g. oauth-hidden-models, whose entries are filtered out of listings only.

3. Same for the per-credential excluded-models.

Current workaround

None that keeps both the catalogue clean and the models callable.