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/lib/walkthrough/types.ts

@@ -91,6 +91,7 @@ export type WalkthroughStage = 'collecting' | 'asking' | 'retrying' | 'assemblin
 
 export type WalkthroughBlockedReason =
   | 'no-model'
+  | 'no-provider-login'
   | 'empty-diff'
   | 'only-generated'
   | 'context-too-small'