Add API Route as a built-in OpenAI-compatible provider preset

[DennyHo0917]

Problem

API Route is OpenAI-compatible and can already be configured manually in Zero,
but users need to enter the endpoint, API key environment variable, and model manually.

A built-in provider preset would make setup easier and allow API Route to appear
in Zero's existing provider catalog and setup flow.

Proposed behavior

Add a focused API Route provider preset using Zero's existing
OpenAI-compatible provider implementation.

Suggested configuration:

• Provider ID: apiroute
• Label: API Route
• Base URL: [填你的 OpenAI-compatible API 地址]
• API key env: API_ROUTE_API_KEY
• Authentication: Bearer
• Reuse Zero's existing OpenAI-compatible chat/tool/streaming runtime
• Reuse /models discovery if supported

No new transport or dependency should be required.

Why this belongs in Zero

API Route provides access to multiple major LLM families through a single
OpenAI-compatible endpoint.

Users can already configure it manually today, so this proposal mainly improves
discoverability and setup UX.

Alternatives

Users can continue using Zero's custom OpenAI-compatible provider configuration,
but that requires manual setup every time.

If the maintainers are open to this, I can prepare a small, focused implementation
with tests and documentation.