[feature] Add OrcaRouter as a named model provider

What problem does this solve?

config.yaml and the Settings UI both treat model endpoints as a generic OpenAI-compatible pair of base_url + model. That works, but for users who route through OrcaRouter — an OpenAI-compatible model gateway with smart routing across many upstream models — it means hand-typing the endpoint every time and re-discovering the right model IDs. The repo already sets the precedent for a named provider (provider: AzureOpenAIChatCompletionClient in src/magentic_ui/_ai_client.py, the model_config_azure_example block in config.yaml.example, and the Azure bullet in docs/configuration.md). A named OrcaRouter provider would mirror that precedent so routing through the gateway is a one-block setup.

It also runs gateway-level, zero-trust security for AI agents on the same endpoint — screening every prompt/response and governing every tool call on a default-deny basis, with no application code changes.

Proposed solution

Add OrcaRouter as a named model provider, mirroring the existing Azure named-provider pattern:

1. Backend registry — src/magentic_ui/_ai_client.py:

   • Add an _ORCAROUTER_PROVIDERS set and a _create_orcarouter() factory alongside _create_azure(), dispatching on provider: OrcaRouterChatCompletionClient.
   • The factory returns the standard AsyncOpenAI client with base_url=https://api.orcarouter.ai/v1, an ORCAROUTER_API_KEY env-var fallback, and a default model of orcarouter/auto (OrcaRouter's smart-routing model). Everything downstream (the ChatClient wrapper, OmniAgent, FaraWebSurfer) is already OpenAI-compatible and needs no changes.

2. Config example — config.yaml.example: add a model_config_orcarouter anchor block for both the orchestrator and web-surfer roles (mirroring the Azure example), so users can route either agent through the gateway.

3. Docs — docs/configuration.md: add an OrcaRouter bullet and YAML example in the "Model clients" notes (mirroring the Azure note), including that the browser-use model should be a vision-capable gateway model (e.g. orcarouter/fusion).

4. Optional UI polish — the model cards in ModelSettings.tsx / onboarding currently accept free-form input; a provider preset that pre-fills https://api.orcarouter.ai/v1 + orcarouter/auto would make adoption one click.

I verified the gateway's OpenAI-compatible surface with a live probe:

• GET https://api.orcarouter.ai/v1/models → 200, 208 models, orcarouter/auto present.
• POST https://api.orcarouter.ai/v1/chat/completions with model: "orcarouter/auto" → 200 with a normal chat.completion response (routed to deepseek-v4-pro), which is exactly the shape MagenticLite's _verify_single_endpoint probe and create_openai_client expect.

Alternatives considered

• Generic OpenAI-compatible passthrough only (current state): already works if users type https://api.orcarouter.ai/v1 by hand, but there's no named entry point, no preset, and no documentation — the endpoint stays invisible and each user re-discovers it. A named provider gives the same one-block config the repo already offers for Azure.
• Frontend-only dropdown: would be user-visible but without the backend provider dispatch the saved config still records a generic client; the backend named provider is the foundation, so it should come first.

Additional context

I'm an engineer on the OrcaRouter team — happy to turn this into a PR (the code is small and mirrors the Azure wiring), and to adjust the model IDs, defaults, or docs based on your preferences. I understand from CONTRIBUTING that code PRs are currently limited to vetted contributors; opening this issue first per the repo's issues-first guidance.