refactor: simplify FormData type checking and fix string handling

- Remove redundant File instanceof check (File extends Blob)
- Add explicit string type check to prevent double-stringification
- Strings now passed directly to FormData instead of JSON.stringify

Thanks to @lc-soft for the code review and suggestions!

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: omarmciver

templates/base/http-clients/fetch-http-client.ejs

@@ -116,8 +116,7 @@ export class HttpClient<SecurityDataType = unknown> {
                 const propertyContent = property instanceof Array ? property : [property];
 
                 for (const formItem of propertyContent) {
-                    const isFileType = formItem instanceof Blob || formItem instanceof File;
-                    formData.append(key, isFileType ? formItem : JSON.stringify(formItem));
+                    formData.append(key, formItem instanceof Blob || typeof formItem === "string" ? formItem : JSON.stringify(formItem));
                 }
                 return formData;
             }, new FormData());