Fix: Correct inverted logic in signUpWithoutWorkspace causing 'User already exists' error (#15086)

## Problem

When trying to signup on localhost:3001 with a new random email, users
were getting an error message saying 'User already exists', even though
they were new users.

## Root Cause

The `signUpWithoutWorkspace` method in `sign-in-up.service.ts` had
inverted logic when checking if a user exists. It was using
`findUserByEmailOrThrow` which:
- **Returns the user** if found
- **Throws the provided error** if NOT found

This caused the opposite behavior:
- ❌ **New user (doesn't exist)**: Threw 'User already exists' error
- ❌ **Existing user**: Continued to create duplicate user

## Solution

Changed to use `findUserByEmail` and explicitly check if the user exists
before throwing the appropriate error:

```typescript
const existingUser = await this.userService.findUserByEmail(newUserParams.email);

if (existingUser) {
  throw new AuthException(
    'User already exist',
    AuthExceptionCode.USER_ALREADY_EXIST,
    { userFriendlyMessage: msg`User already exists` },
  );
}
```

This matches the correct pattern already used in `signUpInWorkspace`
(line 431-441 in auth.resolver.ts).

## Changes
- Fixed inverted logic in `signUpWithoutWorkspace` method
- Now correctly validates that user does NOT exist before creating new
user
- Matches the pattern used in `signUpInWorkspace`

## Testing
The fix corrects the logic so that:
- ✅ New users can sign up successfully
- ✅ Existing users get the correct 'User already exists' error

packages/twenty-server/src/engine/core-modules/auth/auth.exception.ts

@@ -30,5 +30,5 @@ export const AuthExceptionCode = appendCommonExceptionCode({
     'TWO_FACTOR_AUTHENTICATION_PROVISION_REQUIRED',
   TWO_FACTOR_AUTHENTICATION_VERIFICATION_REQUIRED:
     'TWO_FACTOR_AUTHENTICATION_VERIFICATION_REQUIRED',
-  USER_ALREADY_EXIST: 'USER_ALREADY_EXIST',
+  USER_ALREADY_EXISTS: 'USER_ALREADY_EXISTS',
 } as const);

packages/twenty-server/src/engine/core-modules/auth/services/sign-in-up.service.ts

@@ -465,15 +465,18 @@ export class SignInUpService {
     newUserParams: SignInUpNewUserPayload,
     authParams: AuthProviderWithPasswordType['authParams'],
   ) {
-    await this.userService.findUserByEmailOrThrow(
+    const userExists = await this.userService.findUserByEmail(
       newUserParams.email,
-      new AuthException(
-        'User already exist',
-        AuthExceptionCode.USER_ALREADY_EXIST,
-        { userFriendlyMessage: msg`User already exists` },
-      ),
     );
 
+    if (userExists) {
+      throw new AuthException(
+        'User already exists',
+        AuthExceptionCode.USER_ALREADY_EXISTS,
+        { userFriendlyMessage: msg`User already exists` },
+      );
+    }
+
     return this.saveNewUser(
       await this.computePartialUserFromUserPayload(newUserParams, authParams),
       await this.setDefaultImpersonateAndAccessFullAdminPanel(),

packages/twenty-server/src/engine/core-modules/auth/utils/auth-graphql-api-exception-handler.util.ts

@@ -26,7 +26,7 @@ export const authGraphqlApiExceptionHandler = (exception: AuthException) => {
     case AuthExceptionCode.SIGNUP_DISABLED:
     case AuthExceptionCode.MISSING_ENVIRONMENT_VARIABLE:
     case AuthExceptionCode.INVALID_JWT_TOKEN_TYPE:
-    case AuthExceptionCode.USER_ALREADY_EXIST:
+    case AuthExceptionCode.USER_ALREADY_EXISTS:
       throw new ForbiddenError(exception);
     case AuthExceptionCode.GOOGLE_API_AUTH_DISABLED:
     case AuthExceptionCode.MICROSOFT_API_AUTH_DISABLED:

packages/twenty-server/src/engine/core-modules/auth/utils/get-auth-exception-rest-status.util.ts

@@ -22,7 +22,7 @@ export const getAuthExceptionRestStatus = (exception: AuthException) => {
     case AuthExceptionCode.MISSING_ENVIRONMENT_VARIABLE:
     case AuthExceptionCode.EMAIL_NOT_VERIFIED:
     case AuthExceptionCode.INVALID_JWT_TOKEN_TYPE:
-    case AuthExceptionCode.USER_ALREADY_EXIST:
+    case AuthExceptionCode.USER_ALREADY_EXISTS:
       return 403;
     case AuthExceptionCode.TWO_FACTOR_AUTHENTICATION_PROVISION_REQUIRED:
     case AuthExceptionCode.TWO_FACTOR_AUTHENTICATION_VERIFICATION_REQUIRED: