fix : block store when 0 peers

[mateeullahmalik]

No description provided.

[roomote]

Rooviewer   See task

Reviewed all six changed files. The two-layer defense (early PeersCount guard in StoreArtefacts + error return from IterateBatchStore when knownNodes == 0) is sound, and the tests cover the new behavior well. No issues found.

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