fix(auth): clarify error when joining a non-active workspace (#20769)

## Summary

When an existing user accepts an invite into a workspace whose
`activationStatus` is not `ACTIVE` (e.g. `SUSPENDED`, `INACTIVE`,
`PENDING_CREATION`), the throw in
`throwIfWorkspaceIsNotReadyForSignInUp` returns:

> User is not part of the workspace

The message describes the symptom (they aren't a member yet) instead of
the cause (the workspace can't accept new members), which makes invitees
assume their invite is broken when the real issue is the target
workspace's state.

The sibling branch a few lines above — for brand-new users hitting the
same non-ACTIVE workspace — already returns `"Workspace is not ready to
welcome new members"`. This PR reuses the same message in the
existing-user branch so both paths give a consistent, accurate
explanation.

Single file, two string literals.

## Test plan

- [ ] Sign in via Google with an existing Twenty account, accepting an
invite to a `SUSPENDED` workspace → confirm the new message is shown
instead of "User is not part of the workspace".
- [ ] Confirm the happy path (sign-in to an `ACTIVE` workspace via
invite) is unchanged — early-return on `ACTIVE` is untouched.

---------

Co-authored-by: Charles Bochet <charles@twenty.com>

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

@@ -249,10 +249,10 @@ export class SignInUpService {
 
     if (!userWorkspaceExists) {
       throw new AuthException(
-        'User is not part of the workspace',
+        'Workspace is not ready to welcome new members',
         AuthExceptionCode.FORBIDDEN_EXCEPTION,
         {
-          userFriendlyMessage: msg`User is not part of the workspace`,
+          userFriendlyMessage: msg`Workspace is not ready to welcome new members`,
         },
       );
     }