Fix undefined providerId

[Regloom]

☕️ Reasoning

Client-side auth with no providerId param results in error. Sample code:

import { signIn, signOut } from "@auth/sveltekit/client"
<button onclick={() => signIn()}>Sign In</button>

It's caused by parseActionAndProviderId function which returns providerId = "undefined" (as a string). This breaks the following check auth/core/src/lib/utils/providers.ts

export default function parseProviders(params) {
...
    if (providerId && !provider) {
        const availableProviders = providers.map((p) => p.id).join(", ");
        throw new Error(`Provider with id "${providerId}" not found. Available providers: [${availableProviders}].`);
    }
    return { providers, provider };
}

🧢 Checklist

• Documentation
• Tests
• Ready to be merged

🎫 Affected issues

📌 Resources

• Security guidelines
• Contributing guidelines
• Code of conduct
• Contributing to Open Source

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
auth-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 18, 2025 8:11am

1 Skipped Deployment

Name	Status	Preview	Comments	Updated (UTC)
next-auth-docs	⬜️ Ignored (Inspect)	Visit Preview		May 18, 2025 8:11am

[vercel]

@Regloom is attempting to deploy a commit to the authjs Team on Vercel.

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[ThangHuuVu]

thanks for the fix, could you add a test for it?

[Regloom]

Ok, tests added.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 39.19%. Comparing base (53b445f) to head (8488b4b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12947   +/-   ##
=======================================
  Coverage   39.18%   39.19%           
=======================================
  Files         200      200           
  Lines       31612    31615    +3     
  Branches     1374     1381    +7     
=======================================
+ Hits        12388    12391    +3     
  Misses      19224    19224           

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