test(form-builder): bump remaining backfill-test step counts for WS-6 v1.3-delta D1 migration

Same root cause as 832375b — the new failure_response_code migration
sits at the top of the WS-5/WS-6 stack, so every test that pins --step
to walk back through that stack needs +1.

- FormFieldOptionsBackfillTest:     6 -> 7  (10 occurrences)
- ConditionalLogicBackfillTest:    10 -> 11 (4 occurrences)
- FormFieldConfigBackfillAndDropTest: 16 -> 17 (1 occurrence)
- FormFieldValidationRuleBackfillTest: 19 -> 20 (7 occurrences)

Total: 22 backfill tests now green again.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

api/tests/Feature/FormBuilder/Configs/FormFieldConfigBackfillAndDropTest.php

@@ -30,7 +30,7 @@ final class FormFieldConfigBackfillAndDropTest extends TestCase
         // Roll back 4 WS-5c migrations + 2 WS-6 migrations + 5 WS-5b
         // migrations = 11, to get the pre-WS-5b state where the JSON column
         // still exists on form_fields / form_field_library.
-        $this->artisan('migrate:rollback', ['--step' => 16])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 17])->assertSuccessful();
         $this->assertTrue(Schema::hasColumn('form_fields', 'validation_rules'));
 
         $fieldId = $this->seedField([