feat: add custom value support for s3 region (#15968)

### What problem does this PR solve?
Allow S3-compatible data source region fields to accept custom values
while preserving search-and-select behavior.

### Type of change
- [x] New Feature (non-breaking change which adds functionality)

web/src/pages/user-setting/data-source/constant/s3-constant.tsx

@@ -19,6 +19,7 @@ export const S3Constant = (t: TFunction) => [
     type: FormFieldType.Select,
     required: false,
     options: awsRegionOptions,
+    allowCustomValue: true,
     customValidate: (val: string, formValues: any) => {
       const credentials = formValues?.config?.credentials || {};
       const bucketType = formValues?.config?.bucket_type || 's3';