docs: PZERO openai/chat HTTP connector example

Please describe the feature you want

A named PZERO example on https://tabby.tabbyml.com/docs/references/models-http-api/openai/ (chat only). Tabby already supports kind = "openai/chat" with api_endpoint at the /v1 root. Users still guess whether to append /chat/completions.

Chat only. Do not document PZERO as a FIM completion or embedding provider unless you have verified those routes.

[model.chat.http]
kind = "openai/chat"
model_name = "deepseek-v4-flash"
api_endpoint = "https://api.pzero.studio/v1"
api_key = "pzero_YOUR_KEY"

Do not append /chat/completions to api_endpoint.

Get a key: https://pzero.studio/agents (sign-in free; min top-up 1 USDC on Base; no starter credits). Live text ids: GET https://api.pzero.studio/v1/models (public).

I am not asking for a first-party Tabby connector. openai/chat HTTP is the existing seam.

Additional context

Every AI Credit is at least 20% off $1 list. Usage is paid. Do not start implementation on my behalf until the board says Ready; the TOML is for users today.