Partially update an asset (RFC 7386 JSON Merge Patch)

Partially updates an asset using JSON Merge Patch semantics (RFC 7386). Send only the fields you want to change under form_fields: a field present with a value sets it, a field present with null clears it, and an omitted field is left untouched. Unlike PUT, you do not need to resend the entire form. Concurrent updates are guarded by optimistic locking and retried automatically.