Fold one streamed tool-call chunk's fields into the buffer.
A dict args replaces any accumulated fragments (the provider delivered
the whole value at once); a string args is appended as a fragment; any
other non-None value is stored as-is.
The tool name from this chunk, if present.
The tool-call id from this chunk, if present.
The args field from this chunk (dict, string fragment, or
other scalar).
