refactor(snapshots): drop ignored ?network= query param, rely on client filter

[Kewe63]

Summary

The snapshot listing API at https://snapshots.arc.network/api/snapshots returns entries for every network regardless of the ?network= query parameter — client-side filtering by network has been the actual behavior since the parameter was introduced. Carrying the parameter in the URL is noise and, worse, implies a server-side contract that does not exist.

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Fix

• Dropped ?network= from the request URL.
• Tightened the comment (previously a FIXME) to accurately describe the client-side filter as the real filter, and flagged the request URL as the only place that would need to change if the server ever starts honouring ?network=.

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Changes

File: crates/arc-snapshots/src/download.rs

• Removed ?network= query parameter from request URL.
• Updated comment to reflect actual client-side filtering contract.

Tests updated:

Test	Change
fetch_latest_snapshot_urls_returns_correct_urls	No longer pins the server to a specific ?network= value
fetch_latest_snapshot_urls_uses_devnet_network_param	Renamed to fetch_latest_snapshot_urls_uses_devnet_listing; reworked to assert the actual contract — devnet chain is selected correctly from a multi-network listing
fetch_latest_snapshot_urls_filters_by_network_devnet	New symmetric test for the devnet filter, mirroring the existing testnet one

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How to Test

# Snapshot download tests
cargo test -p arc-snapshots --lib download::
# ✅ 32 passed, 0 failed

# Format & lint
cargo fmt --check -p arc-snapshots
cargo clippy -p arc-snapshots --all-targets -- -D warnings
# ✅ Both clean

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Risk & Impact

None. The server never honored ?network=, so removing it from the request changes nothing at runtime. Client-side filtering behavior is unchanged. Tests now accurately reflect the actual contract.

Type: ♻️ Refactor