fix: normalize fractional credential expiry timestamps

[vupv]

Summary

• truncate fractional OAuth expires_in values to integer epoch milliseconds
• normalize fractional expiry timestamps already stored in Keychain or the credentials file
• add regression coverage for both credential ingress paths

Root cause

OpenCode validates OAuth expires as a non-negative integer. A fractional expires_in from the OAuth endpoint produced a fractional expiresAt, which the plugin persisted unchanged to Keychain and auth.json. OpenCode then ignored the Anthropic credential.

Testing

• pnpm test
• pnpm run lint
• pnpm run build