Support model providers in the app-server model/list API

What variant of Codex are you using?

app-server API through my Giskard web frontend

What feature would you like to see?

Problem

The app-server model/list API currently lists models only for the configured
default provider. Unlike thread/start, clients cannot select a model provider.

This makes it difficult for clients to present models for another configured
provider before starting a thread.

Proposed behavior

Add an optional modelProvider parameter to model/list:

{
  "modelProvider": "amazon-bedrock",
  "limit": 100,
  "cursor": null,
  "includeHidden": false
}

When omitted, model/list should continue using the configured default provider.

The response should also identify the provider associated with the returned
catalog. Two possible shapes are:

1. Add modelProvider to every model, allowing future mixed-provider responses.
2. Add modelProvider to ModelListResponse, establishing that one response
   always contains models from exactly one provider.

The response-level field may be preferable because the request resolves exactly
one provider and avoids repeating identical metadata:

{
  "modelProvider": "amazon-bedrock",
  "data": [
    {
      "id": "openai.gpt-5.6-sol",
      "model": "openai.gpt-5.6-sol"
    }
  ],
  "nextCursor": null
}

Listing models from all providers could be considered separately. That design
would need explicit rules for ordering, pagination, per-provider defaults,
latency, and partial failures.

Implementation considerations

• Unknown provider IDs should return an invalid-request error.
• Provider resolution should use the latest effective configuration.
• Pagination cursors should remain scoped to the selected provider.
• Provider-specific model managers should be reused in memory.
• Alternate providers should not share the current file-backed model cache,
  because that cache is not partitioned by provider.

Reference implementation

I explored an implementation on my fork:

https://github.com/marmeladema/codex/tree/model-list-provider-support

The reference implementation uses the per-model modelProvider shape. It also
includes generated API schemas, documentation, and integration coverage for
explicit provider selection, default-provider fallback, and unknown providers.

I understand that external pull requests are not accepted; the fork is provided
only as technical exploration and a concrete reference for the design.

Additional information

No response

Start with the app-server model/list API and the generated API schemas, then compare the existing thread/start provider selection behavior. Review the documentation and integration coverage described in the reference implementation. Done means explicit provider selection, default-provider fallback, invalid unknown-provider handling, provider-scoped pagination, and a response that identifies its provider.