Add comprehensive unit test coverage for DNS-over-gRPC and
DNS-over-QUIC server implementations:
- server_grpc_test.go: Tests gRPC server creation, TLS config,
lifecycle methods, Query handling, and response writer
- server_quic_test.go: Tests QUIC server creation, custom limits,
message validation, DOQ message parsing, and writer interface
Tests focus on component-level validation with mocks,
complementing existing integration tests without overlap.
Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>