Fix KeyConfig default capabilities

[pollychen-lab]

Summary

Fixes #147.

KeyConfig documented encryption and decryption as enabled by default, and KeyConfig::new() already set both flags to true, but the derived Default implementation left all booleans as false. This made KeyConfig::default() create a key configuration that could neither encrypt nor decrypt.

This PR replaces the derived default with an explicit Default implementation that delegates to KeyConfig::new(), keeping the idiomatic constructor aligned with the documented contract. It also adds a regression test for the expected default capabilities.

Validation

• cargo fmt --check
• cargo test -p egide-transit test_key_config_default_matches_new
• cargo test -p egide-transit
• cargo clippy -p egide-transit --all-targets -- -D warnings