Raised when a provider is selected but its API key env var is unset.
Subclasses ModelConfigError so existing except ModelConfigError blocks
keep working. Carries the provider name and the canonical env_var so
callers can render targeted recovery hints (e.g., "set OPENAI_API_KEY" or
"run /model <other_provider>:<model>") without string-matching on the
formatted exception message and without re-deriving the env-var name.
Human-readable message describing the missing credential.
The provider whose credentials are missing
(e.g., 'openai').
The canonical env var name expected to hold the
credential (e.g., 'OPENAI_API_KEY'). None when the
provider has no registered env-var mapping.
