feature: Custom AI for Chat Completions-only hosts (OpenAI provider uses Responses)

The OpenAI AI provider builds {base_url}/responses (backend/windmill-ai/src/providers/openai.rs). Custom AI builds {base_url}/chat/completions (providers/other.rs). Two details are easy to get wrong on a Chat Completions-first host:

1. Do not pick the OpenAI provider and paste a custom base URL. That wire is Responses.
2. Custom AI base URL is the /v1 root. The code appends /chat/completions. Do not paste the chat path.

PZERO is a live OpenAI-compatible host. Catalog: GET https://api.pzero.studio/v1/models. Default text id: deepseek-v4-flash. Prepaid Bearer key (pzero_…). POST /v1/responses exists (401 without a key, not 404) but Chat Completions is the working path for agent tools.

Custom AI:

• Base URL: https://api.pzero.studio/v1
• API key: pzero_…
• Model: deepseek-v4-flash

No first-party provider asked. If a named vendor subsection is too much, OpenAI=Responses vs Custom AI=Chat Completions is the docs gap.