fix: fix sendCode's concurrency code.

src/main/kotlin/com/waffletoy/team1server/account/controller/UserController.kt

@@ -8,7 +8,6 @@ import com.waffletoy.team1server.account.service.UserService
 import io.swagger.v3.oas.annotations.Parameter
 import jakarta.servlet.http.HttpServletResponse
 import org.springframework.beans.factory.annotation.Value
-import org.springframework.http.HttpStatus
 import org.springframework.http.ResponseCookie
 import org.springframework.http.ResponseEntity
 import org.springframework.web.bind.annotation.*
@@ -24,17 +23,19 @@ class UserController(
     @PostMapping("/signup/send-code")
     fun sendCode(
         @RequestBody request: SendCodeRequest,
-    ): ResponseEntity<Void> {
-        CompletableFuture.runAsync {
+    ): CompletableFuture<ResponseEntity<Void>> {
+        return CompletableFuture.runAsync {
             emailService.sendCode(request.snuMail)
-        }.exceptionally {
-            throw EmailServiceException(
-                "동일한 스누메일로 등록된 계정이 존재합니다.",
-                HttpStatus.CONFLICT,
-            )
-        }.join()
-
-        return ResponseEntity.ok().build()
+        }.handle { _, ex ->
+            if (ex != null) {
+                val cause = ex.cause ?: ex
+                if (cause is EmailServiceException) {
+                    throw cause // Propagate the original EmailServiceException
+                }
+                throw RuntimeException("Unexpected error occurred", cause) // Wrap other exceptions
+            }
+            ResponseEntity.ok().build() // Return the successful response
+        }
     }
 
     // 유저 이메일 인증 링크 클릭