Fix hang when batch_size=0 by validating input

[Mihirmaru22]

This PR fixes an infinite hang in Batcher when batch_size=0 by validating input at construction time.

Previously, batch_size=0 caused Batcher.next() to repeatedly return empty batches without raising StopIteration.
This change raises a ValueError for invalid batch_size values and adds a regression test.

Fixes #1463

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[divyanshk]

LGTM - thanks for fixing this! I'll kick off the CI - when everything passes, will merge the PR in.