Remove the early ValidateEnvironment gate from CreateProvider that only
checked env vars and --provider-api-key, blocking stored OAuth credentials
from working. Each provider creation function already handles its own auth
resolution with clear error messages.
Update ValidateEnvironment to also check stored Anthropic credentials so
the model selector UI correctly shows Anthropic models for OAuth users.
Add automatic token refresh in oauthTransport so long-lived ACP sessions
survive token renewals. Surface actionable auth error messages in ACP
session creation.
Fix pre-existing staticcheck SA5011 warnings in test files.
Ed Zynda
a05da5f3ab