opencode-zen serves the public, signup-free OpenCode Zen endpoint
(https://opencode.ai/zen/v1). With no API-key connection configured,
getProviderCredentials returned null, so the Playground/combos surfaced
"No credentials for provider: opencode-zen" when selecting an OpenCode free
model.
Fall back to synthetic no-auth credentials for opencode-zen when no usable
connection exists (a configured active key is still selected first; a
rate-limited/terminal key returns its own signal before this point). Other
api-key providers are unaffected.
Closes#2962
