[Fix] upstream: fix stale heap_idx in token bucket

[Fix] upstream: fix stale heap_idx in token bucket

The intrusive heap swaps entire structs during swim/sink, making
up->heap_idx stale after any heap modification. The update function
would silently skip updates when the cached index pointed to a
different upstream, breaking load distribution across backends.

Fix by falling back to linear search on cache miss and refreshing
heap_idx after every heap update. Also add underflow warning for
double-return detection and improve API documentation.