fix: remove duplicate OIDC provider registration in getSocialProviders (#475)

The getSocialProviders endpoint was manually pushing "oidc" to the 
providers array when OIDC env vars are present, but Better Auth already 
includes the OIDC provider in ctx.context.socialProviders when it is 
configured. This resulted in ["oidc","oidc"] being returned from 
/api/auth/social-providers, causing a duplicate provider registration 
that led to a TypeError: Cannot read properties of null (reading 'id') 
during the OIDC login callback, preventing all OIDC logins.

Fix: remove the manual providers.push("oidc") block as it is redundant.

packages/auth/src/providers.ts

@@ -92,14 +92,6 @@ export const socialProvidersPlugin = () => ({
         const providers = ctx.context.socialProviders.map((p) =>
           p.id.toLowerCase(),
         );
-        // Add OIDC provider if configured
-        if (
-          process.env.OIDC_CLIENT_ID &&
-          process.env.OIDC_CLIENT_SECRET &&
-          process.env.OIDC_DISCOVERY_URL
-        ) {
-          providers.push("oidc");
-        }
         return ctx.json(providers);
       },
     ),