fix: map Convex table names to Better Auth model keys in isUniqueField

Fixes a bug where isUniqueField tries to access the Better Auth schema
using Convex table names (e.g., "users") directly, but the schema object
uses Better Auth's internal model names as keys (e.g., "user").

This causes the error: "Cannot read properties of undefined (reading 'fields')"

The fix maps Convex table names to Better Auth internal model names by
finding the key where betterAuthSchema[key].modelName === model, ensuring
compatibility with custom modelName values in Better Auth configuration.

Author: thuillart

src/adapterUtils.ts

@@ -62,14 +62,20 @@ const isUniqueField = (
   model: string,
   field: string
 ) => {
-  const fields =
-    betterAuthSchema[model as keyof typeof betterAuthSchema].fields;
-
-  if (!fields) {
+  // Map Convex table name (e.g., "users") to Better Auth model key (e.g., "user")
+  // by finding the key where betterAuthSchema[key].modelName === model
+  const betterAuthModel =
+    Object.keys(betterAuthSchema).find(
+      (key) => betterAuthSchema[key as keyof typeof betterAuthSchema].modelName === model
+    ) || model;
+  const modelSchema =
+    betterAuthSchema[betterAuthModel as keyof typeof betterAuthSchema];
+
+  if (!modelSchema?.fields) {
     return false;
   }
 
-  return Object.entries(fields)
+  return Object.entries(modelSchema.fields)
     .filter(([, value]) => value.unique)
     .map(([key]) => key)
     .includes(field);