fix(walkthrough): block unauthenticated providers with a friendly refusal

When the walkthrough small model resolves to a provider with no usable
login, readiness was still ready and generate returned a raw 500 message.
Refuse up front with no-provider-login and surface a blocker instead.

Closes openchamber/openchamber#2607

Co-authored-by: Serhii Dziupin <makeittech@users.noreply.github.com>

packages/ui/src/components/views/walkthrough/WalkthroughView.tsx

@@ -406,6 +406,7 @@ export const WalkthroughView = ({ directory }: WalkthroughViewProps) => {
   const blockedReason = entry.error?.code === 'context-too-small'
     || entry.error?.code === 'structured-output-unsupported'
     || entry.error?.code === 'no-model'
+    || entry.error?.code === 'no-provider-login'
     || entry.error?.code === 'empty-diff'
     || entry.error?.code === 'only-generated'
     || entry.error?.code === 'output-exhausted'