Batch multiple Meta Marketing API mutations into a single Graph API batch request

Steps

1. POST to the root Graph API endpoint with a top-level access_token and a batch parameter containing a JSON array of {method, relative_url, body} objects, up to 50 requests per batch.
2. Reference a prior operation's output in a later operation using {result=<name>:$.<JSONPath>} syntax, after tagging the earlier operation with a name field.
3. Optionally set include_headers=false to shrink the response payload when header data isn't needed.
4. Attach binary uploads (e.g. image creatives) via multipart/mime with the attached_files field listing comma-separated attachment names per operation.
5. Parse the response array in request order; independent operations are processed in parallel while dependent ones run sequentially, and each entry has its own status code and body.