fix(download): reject zero blocks_per_file in snapshot manifest

[Aditya-9-6]

Description

Resolves #27030

A modular snapshot manifest with blocks_per_file: 0 triggers a divide-by-zero panic in ChunkedArchive::num_chunks() (self.total_blocks.div_ceil(self.blocks_per_file)) when calculating chunk counts and expanding snapshot archives.

This PR addresses this by:

1. Adding SnapshotManifest::validate(&self) to check that any chunked component specifies blocks_per_file > 0.
2. Calling manifest.validate()? early in DownloadCommand::load_manifest() right after verifying chain_id.
3. Adding an explicit guard in collect_planned_archives() before chunk consistency verification.
4. Adding defensive zero guards in ChunkedArchive::num_chunks() and ChunkedArchive::tail_chunks_for_distance(), plus saturating arithmetic in ChunkedArchive::chunk_relative_path().
5. Adding unit tests for manifest validation and non-panicking ChunkedArchive behavior.