runtime, graph: fix VID collisions in ipfs.map() callbacks

Each ipfs.map() callback created a fresh BlockState with vid_seq reset
to RESERVED_VIDS. When multiple callbacks write to the same entity
table, they all generate vids starting at (block<<32)+100, producing
duplicates.

Fix by threading vid_seq through the callback loop and updating it in
EntityCache::extend() so the parent handler also continues from the
right sequence after merging callback results.

Addresses same underlying issue as PR https://github.com/graphprotocol/graph-node/pull/6336