`pglite-sync` saves incorrect `offset` for shapes returning more than one chunk

(+ some discussion on discord).

According to @thruflo , "The electric-offset header "Must be used as the value of the offset parameter in your next request."
https://electric-sql.com/openapi.html#/paths/~1v1~1shape/get". `pglite-sync` currently uses `messageAggregator[messageAggregator.length - 1].offset` https://github.com/electric-sql/pglite/blob/6b60fbc55c2d59eb7642d1d3f560999ec5b4ec40/packages/pglite-sync/src/index.ts#L252

As all operation-level `offset`s appear to be the same, regardless of what is returned by the `electric-offset` header, that means `pglite-sync` will save the incorrect `offset` for any shape that returns more than one chunk initially.

What appears needs doing is to keep the last valid `Offset` returned by `stream.lastOffset` (so ignore `0_inf`), which you can get at the moment you receive the last chunk.