Experimental chunk iterator factory 2

[tcp13equals2]

What this PR does

This is an experimental build which pulls in grafana/mimir-prometheus#1106 over bot/main/update-mimir-prometheus-8aa312f6fd75-202602160717.

The changes made to use the new IteratorFactory() are then;

pkg/ingester/ingester.go

• chunkSeriesNode.series: changed from []storage.ChunkSeries to []storage.ChunkIterable
• getChunkSeriesNode(): allocates []storage.ChunkIterable
• sendStreamingQuerySeries(): stores series.IteratorFactory() instead of the full series, and updated the comment to explain the rationale
• sendStreamingQueryChunks(): unchanged — storage.ChunkIterable has the same Iterator(chunks.Iterator) chunks.Iterator method, so iterator reuse works identically

pkg/ingester/mirror_querier_test.go

• Added IteratorFactory() to mockChunkSeries to satisfy the updated storage.ChunkSeries interface (which now embeds ChunkIterableFactory in the new prometheus commit)

Which issue(s) this PR fixes or relates to

Fixes #

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]. If changelog entry is not needed, please add the changelog-not-needed label to the PR.
• about-versioning.md updated with experimental features.