test(config): add unit tests for ExecutionConfig, NetworkConfig, and RpcConfig

[refcell]

Summary

• Add comprehensive unit tests to ExecutionConfig, NetworkConfig, and RpcConfig modules
• Increases kora-config test coverage from 18 to 37 tests

New Tests

ExecutionConfig (6 tests)

• test_default_execution_config: Verifies default values match constants
• test_execution_config_serde_roundtrip: JSON serialization/deserialization
• test_execution_config_toml_roundtrip: TOML serialization/deserialization
• test_execution_config_serde_defaults: Empty JSON uses correct defaults
• test_execution_config_partial_defaults: Partial JSON preserves defaults for missing fields
• test_execution_config_clone_and_eq: Clone and equality implementations

NetworkConfig (7 tests)

• test_default_network_config: Verifies default values and empty collections
• test_network_config_serde_roundtrip: JSON serialization/deserialization
• test_network_config_toml_roundtrip: TOML serialization/deserialization
• test_network_config_serde_defaults: Empty JSON uses correct defaults
• test_network_config_dialable_addr_skip_serializing_when_none: Optional field omission
• test_network_config_dialable_addr_serialized_when_some: Optional field inclusion
• test_network_config_clone_and_eq: Clone and equality implementations

RpcConfig (6 tests)

• test_default_rpc_config: Verifies default HTTP/WS addresses
• test_rpc_config_serde_roundtrip: JSON serialization/deserialization
• test_rpc_config_toml_roundtrip: TOML serialization/deserialization
• test_rpc_config_serde_defaults: Empty JSON uses correct defaults
• test_rpc_config_partial_defaults: Partial JSON preserves defaults for missing fields
• test_rpc_config_clone_and_eq: Clone and equality implementations