Compute a stable key for buffering an in-progress streamed tool call.
Prefers the streaming index (stable across fragments of one call), then
the tool-call id, falling back to a positional placeholder so unrelated
id-less calls don't collide.
The index field from the streamed tool-call chunk, if any.
Typed loosely because it is read from an untyped content-block dict.
The tool-call id from the chunk, if any.
The current number of buffered calls, used to make the fallback placeholder unique.
