fix: Signup from OAuth not being detected correctly (appsmithorg#37697)

Fixes issue where the detection for signup when using OAuth was not
being handled correctly.

[Slack
conversation](https://theappsmith.slack.com/archives/C02K2MZERSL/p1732600773587469?thread_ts=1732554015.110689&cid=C02K2MZERSL).


## Automation

/test sanity

### 🔍 Cypress test results
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> [!WARNING]
> Workflow run:
<https://github.com/appsmithorg/appsmith/actions/runs/12024883331>
> Commit: d53fcdf
> <a
href="https://internal.appsmith.com/app/cypress-dashboard/rundetails-65890b3c81d7400d08fa9ee5?branch=master&workflowId=12024883331&attempt=1"
target="_blank">Cypress dashboard</a>.
> Tags: @tag.Sanity
> Spec: 
> It seems like **no tests ran** 😔. We are not able to recognize it,
please check <a
href="https://github.com/appsmithorg/appsmith/actions/runs/12024883331"
target="_blank">workflow here</a>.
> <hr>Tue, 26 Nov 2024 06:16:02 UTC
<!-- end of auto-generated comment: Cypress test results  -->


## Communication
Should the DevRel and Marketing teams inform users about this change?
- [ ] Yes
- [x] No


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **New Features**
- Improved handling of user authentication success, enhancing the flow
for email verification and OAuth2 authentication.
- **Refactor**
	- Simplified the logic for determining user sign-up or login status.
- Streamlined the method for handling OAuth2 redirects, improving
clarity and maintainability.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

app/server/appsmith-server/src/main/java/com/appsmith/server/authentication/handlers/ce/AuthenticationSuccessHandlerCE.java

@@ -252,8 +252,7 @@ public Mono<Void> onAuthenticationSuccess(
             // creation) or if this was a login (existing user). What we do here to identify this, is an approximation.
             // If and when we find a better way to do identify this, let's please move away from this approximation.
             // If the user object was created within the last 5 seconds, we treat it as a new user.
-            final boolean oauthIsFromSignup =
-                    user.getCreatedAt().isAfter(Instant.now().minusSeconds(5));
+            isFromSignup = user.getCreatedAt().isAfter(Instant.now().minusSeconds(5));
 
             // Check the existing login source with the authentication source and then update the login source,
             // if they are not the same.
@@ -270,26 +269,28 @@ public Mono<Void> onAuthenticationSuccess(
                         .subscribeOn(Schedulers.boundedElastic())
                         .subscribe();
             }
-            if (oauthIsFromSignup) {
+            if (isFromSignup) {
+                final boolean isFromSignupFinal = isFromSignup;
                 redirectionMono = workspaceServiceHelper
                         .isCreateWorkspaceAllowed(TRUE)
                         .flatMap(isCreateWorkspaceAllowed -> {
                             if (isCreateWorkspaceAllowed.equals(Boolean.TRUE)) {
                                 return createDefaultApplication(defaultWorkspaceId, authentication)
                                         .flatMap(application -> handleOAuth2Redirect(
-                                                webFilterExchange, application, oauthIsFromSignup));
+                                                webFilterExchange, application, isFromSignupFinal));
                             }
-                            return handleOAuth2Redirect(webFilterExchange, null, oauthIsFromSignup);
+                            return handleOAuth2Redirect(webFilterExchange, null, isFromSignupFinal);
                         });
             } else {
-                redirectionMono = handleOAuth2Redirect(webFilterExchange, null, oauthIsFromSignup);
+                redirectionMono = handleOAuth2Redirect(webFilterExchange, null, isFromSignup);
             }
         } else {
             // form type signup/login handler
             redirectionMono = formEmailVerificationRedirectionHandler(
                     webFilterExchange, defaultWorkspaceId, authentication, isFromSignup, createDefaultApplication);
         }
 
+        final boolean isFromSignupFinal = isFromSignup;
         Mono<Void> finalRedirectionMono = redirectionMono;
         return sessionUserService
                 .getCurrentUser()
@@ -307,7 +308,7 @@ public Mono<Void> onAuthenticationSuccess(
                         modeOfLogin = ((OAuth2AuthenticationToken) authentication).getAuthorizedClientRegistrationId();
                     }
 
-                    if (isFromSignup) {
+                    if (isFromSignupFinal) {
                         final String inviteToken = currentUser.getInviteToken();
                         final boolean isFromInvite = inviteToken != null;