fix: handle projectUserNotFoundErrors and display the error message

[ryanbonial]

Description

This PR adds utility functions for handling client errors in the authentication flow. It exports several functions from the auth/utils module to help extract and interpret error information from ClientError objects. The main additions include:

• Added a function to specifically detect "projectUserNotFoundError" errors
• Updated the LoginError component to use these new utilities for better error handling
• Improved the user experience by conditionally showing retry CTAs based on the error type
• Exported utility functions for extracting API error bodies, types, and descriptions from ClientError objects

What to review

• The new utility functions in auth/utils.ts for handling ClientError objects
• The exports added to the core package
• The implementation in LoginError component that uses these utilities to provide better error messages
• The conditional rendering of the retry CTA based on error type

Testing

Test by:

1. Change the Kitchensink config to { projectId: 'txacgfm1', dataset: 'production' },
2. Set verifyOrganization={false} prop on the SDKProvider component in the SanityApp
3. Run pnpm run deploy:kitchensink
4. Visit https://www.sanity.io/@oblZgbTFj/application/wkyoigmzawwnnwx458zgoh46/document-list
5. Observe error message instead of endless rerendering and token refreshing

Fun gif

[vercel]

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Project	Deployment	Preview	Comments	Updated (UTC)
sdk-docs	Ready	Preview	Comment	Oct 22, 2025 8:29pm
sdk-kitchensink-react	Ready	Preview	Comment	Oct 22, 2025 8:29pm

[cngonzalez]

Code itself looks fine to me.

Not really relevant to this PR, I don't think, but I was following your steps to verify, and changed the devConfigs to the project you mentioned. I deployed at that point, and did get the project user not found for user ID "gdaoiCJFz" in project "txacgfm1".

There's also mention of adding a verifyOrganization=false to the SDKProvider in Kitchensink. As far as I can tell there is no SDKProvider in Kitchensink. I added it manually, something like the below in App.tsx:

    <ThemeProvider theme={theme}>
      <SanityApp fallback={<Spinner />} config={devConfigs}>
        <SDKProvider config={devConfigs} fallback={<Spinner />} verifyOrganization={false}>
        // ...etc

and re-deployed. I ended up getting this error: Project txacgfm1 belongs to Organization oSyH1iET5, but the Dashboard has Organization oblZgbTFj selected

Which is the opposite of what I would expect. Not a blocker for this PR, just making a note of it.