feat: improve error key

Author: kenjis

system/Validation/Validation.php

@@ -356,8 +356,14 @@ protected function processRules(
 
                 $param = ($param === false) ? '' : $param;
 
+                if ($rule === 'field_exists') {
+                    $fieldForErrors = $originalField;
+                } else {
+                    $fieldForErrors = $field;
+                }
+
                 // @phpstan-ignore-next-line $error may be set by rule methods.
-                $this->errors[$field] = $error ?? $this->getErrorMessage(
+                $this->errors[$fieldForErrors] = $error ?? $this->getErrorMessage(
                     ($this->isClosure($rule) || $arrayCallable) ? (string) $i : $rule,
                     $field,
                     $label,

tests/system/Validation/RulesTest.php

@@ -948,8 +948,7 @@ public function testFieldExistsErrorMessage(): void
 
         $this->assertFalse($this->validation->run($data));
         $this->assertSame(
-            // This errror message is not perfect.
-            ['fiz.bar.baz' => 'The fiz.*.baz field must exist.'],
+            ['fiz.*.baz' => 'The fiz.*.baz field must exist.'],
             $this->validation->getErrors()
         );
     }