feat(config, llm): Read an API key from a list of env vars

[JeanMertz]

api_key_env accepts a list of environment variables. The list is one key: JP reads the variables in order and uses the first one holding a non-empty value. A blank value falls through to the next variable, as it already falls through between auth chain entries.

[providers.llm.vllm]
api_key_env = ["WORK_KEY", "USER_KEY"]

A named key in the map form may be a list too. api_key:work then tries each of its variables before the auth chain moves on to the next entry, and still reports the credential as api_key:work:

api_key_env = { work = ["WORK_KEY", "WORK_KEY_OLD"], personal = "KEY" }

When nothing in a list is set, the error names every variable tried (Missing environment variable: WORK_KEY or USER_KEY). A named key with an empty list is reported by its name rather than as a missing variable with no name. --cfg accepts both shapes in JSON form, e.g. --cfg 'providers.llm.vllm.api_key_env:=["A","B"]', where it rejected arrays before. jp provider ls lists one api_key row per variable, in the order they are tried, so it shows which one is set.

Applies to every provider with an api_key_env: Anthropic, Cerebras, DeepSeek, Google, OpenAI, OpenRouter, and vLLM. Existing string and map configs are unchanged.