Return bank existence lookup errors to callers

- Make `bank_exists` surface backend/UID resolution failures instead of returning `false`
- Add SQF guard to log and alert on unexpected existence responses
- Prevent duplicate/default bank records after failed lookups

arma/server/addons/bank/functions/fnc_initBankStore.sqf

@@ -328,6 +328,11 @@ GVAR(BankBaseStore) = compileFinal createHashMapFromArray [
             GVAR(BankMessenger) call ["sendAlert", [_uid, "error", _result select [7]]];
             createHashMap
         };
+        if !(_result in ["true", "false"]) exitWith {
+            ["ERROR", format ["Bank exists check for %1 returned unexpected response: %2", _uid, _result]] call EFUNC(common,log);
+            GVAR(BankMessenger) call ["sendAlert", [_uid, "error", "Bank backend returned an unexpected existence response."]];
+            createHashMap
+        };
 
         private _finalAccount = createHashMap;
         private _wasCreated = false;

arma/server/extension/src/bank.rs

@@ -548,6 +548,8 @@ pub fn update_bank(call_context: CallContext, key: String, json_update: String)
 /// Checks if an bank exists in the database.
 ///
 /// Returns "true" if the bank exists, "false" otherwise.
+/// Backend failures are returned as errors so callers do not confuse a failed
+/// lookup with a missing account and create duplicate/default records.
 pub fn bank_exists(call_context: CallContext, key: String) -> String {
     log(
         "bank",
@@ -561,12 +563,9 @@ pub fn bank_exists(call_context: CallContext, key: String) -> String {
             uid
         }
         None => {
-            log(
-                "bank",
-                "WARN",
-                &format!("Failed to resolve UID for key: {}", key),
-            );
-            return "false".to_string();
+            let error_msg = format!("Error: Failed to resolve UID for key: {}", key);
+            log("bank", "ERROR", &error_msg);
+            return error_msg;
         }
     };
 
@@ -580,12 +579,13 @@ pub fn bank_exists(call_context: CallContext, key: String) -> String {
             exists.to_string()
         }
         Err(e) => {
+            let error_msg = format!("Error: {}", e);
             log(
                 "bank",
                 "ERROR",
                 &format!("Failed to check if bank '{}' exists: {}", resolved_uid, e),
             );
-            "false".to_string()
+            error_msg
         }
     }
 }