fix: fix invite flow while inviting via admin settings (#27383)

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## Description
the logic for handling user invitation on admin settings page was
causing double emails on admin settings page. turns out, we cannot use
isEnabled flag because that flag can also be set by other serviecs for
an existing user.

#### PR fixes following issue(s)
Fixes # (issue number)
no issue - it is for a fix for existing buggy code in release
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#### Type of change
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- Bug fix (non-breaking change which fixes an issue)
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## Testing
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#### How Has This Been Tested?
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- [x] Manual
- [x] JUnit
- [ ] Jest
- [ ] Cypress
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#### Test Plan
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#### Issues raised during DP testing
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## Checklist:
#### Dev activity
- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my own code
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app/server/appsmith-server/src/main/java/com/appsmith/server/solutions/ce/EnvManagerCEImpl.java

@@ -49,6 +49,8 @@ import org.springframework.util.StringUtils;
 import org.springframework.web.server.ServerWebExchange;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
+import reactor.util.function.Tuple2;
+import reactor.util.function.Tuples;
 
 import java.io.File;
 import java.io.FileInputStream;
@@ -591,20 +593,22 @@ public class EnvManagerCEImpl implements EnvManagerCE {
                 .collectList()
                 .flatMap(userUtils::removeSuperUser);
 
-        Flux<User> usersFlux = Flux.fromIterable(newUsers)
-                .flatMap(email -> userService.findByEmail(email).map(existingUser -> {
-                    if (existingUser == null) {
-                        User newUser = new User();
-                        newUser.setEmail(email);
-                        newUser.setIsEnabled(false);
-                        return newUser;
-                    }
-                    return existingUser;
-                }))
+        Flux<Tuple2<User, Boolean>> usersFlux = Flux.fromIterable(newUsers)
+                .flatMap(email -> userService
+                        .findByEmail(email)
+                        .flatMap(user -> {
+                            return Mono.just(Tuples.of(user, false));
+                        })
+                        .switchIfEmpty(Mono.defer(() -> {
+                            User newUser = new User();
+                            newUser.setEmail(email);
+                            newUser.setIsEnabled(false);
+                            return Mono.just(Tuples.of(newUser, true));
+                        })))
                 .cache();
 
-        Flux<User> newUsersFlux = usersFlux.filter(user -> !user.isEnabled());
-        Flux<User> existingUsersFlux = usersFlux.filter(User::isEnabled);
+        Flux<User> newUsersFlux = usersFlux.filter(Tuple2::getT2).map(Tuple2::getT1);
+        Flux<User> existingUsersFlux = usersFlux.filter(tuple -> !tuple.getT2()).map(Tuple2::getT1);
 
         // we are sending email to existing users who are not already super-users
         Mono<List<User>> existingUsersWhichAreNotAlreadySuperUsersMono = existingUsersFlux