Add coverage for backend edge cases

[tripledoublev]

Summary

• add hermetic unit coverage for malformed URLs, DHT parsing, ciphertext envelopes, permissions, repo-slot collision handling, and RPC response parsing
• harden URL/key/hash parsing and encrypted payload handling so bad inputs return errors instead of panicking
• make group repo advertisement/load logic sparse-slot aware and keep RPC list/remove group IDs roundtrippable

Validation

• cargo fmt
• cargo test --lib group::tests
• cargo test --lib repo::tests
• cargo test --lib parse_remove_group_id
• cargo test --lib listed_group_id_roundtrips_through_remove_parser
• timeout 120 cargo test --lib refresh_group_reloads_group_after_cache_invalidation -- --nocapture
• cargo test --lib --no-run
• cargo check
• git diff --check

Full cargo test --lib was not run because the existing suite is network-heavy and slow.