test(dkg): add comprehensive unit tests for DkgConfig type

[refcell]

Summary

Adds comprehensive unit tests for the DkgConfig struct accessor methods.

Tests Added

• test_n_returns_participant_count: Verifies n() returns the correct number of participants
• test_n_with_single_participant: Edge case with single participant
• test_n_with_many_participants: Edge case with 100 participants
• test_t_returns_threshold: Verifies t() returns the correct threshold value
• test_t_with_threshold_one: Edge case with threshold of 1
• test_t_with_large_threshold: Edge case with threshold of 67
• test_my_public_key_derived_from_identity: Verifies public key derivation from identity key
• test_my_public_key_consistent: Verifies consistency across multiple calls
• test_my_public_key_different_identities: Different identities produce different public keys
• test_dkg_config_debug_implementation: Tests Debug trait implementation
• test_dkg_config_clone: Tests Clone trait implementation
• test_dkg_config_participants_matches_public_keys: Verifies participants vector matches accessor
• test_dkg_config_threshold_boundary: Tests threshold == n boundary condition

Verification

✅ All 13 new tests pass
✅ Clippy passes with no warnings
✅ No modifications to non-test code
✅ Follows existing test patterns from the crate