Add proto serialization round-trip tests

[lanycrost]

Problem statement

There are no tests that validate protobuf serialization/deserialization for the generated Go code. While the generated code is produced by well-tested protoc plugins, round-trip tests serve as regression guards for:

• Oneof field behavior (frame with audio/video/binary)
• Reserved field handling (fields 4, 5 in FlowControl)
• google.protobuf.Struct serialization with complex JSON
• Empty vs zero-value vs nil semantics
• Large byte payload handling

Proposed change

Add a new test file gen/go/proto/transport/v1/proto_test.go (or proto_roundtrip_test.go):

func TestDataRequest_RoundTrip_AudioFrame(t *testing.T) {
    req := &transportv1.DataRequest{
        Frame: &transportv1.DataRequest_Audio{
            Audio: &transportv1.AudioFrame{
                Pcm:          []byte("audio-data"),
                SampleRateHz: 48000,
                Channels:     2,
                Codec:        "opus",
                TimestampMs:  1234567890,
            },
        },
    }
    data, err := proto.Marshal(req)
    // ... unmarshal and compare
}

func TestDataRequest_RoundTrip_AllFrameTypes(t *testing.T) { ... }
func TestControlRequest_AllActions(t *testing.T) { ... }
func TestFlowControl_ReservedFieldsNotGenerated(t *testing.T) { ... }
func TestStreamEnvelope_ChunkingFields(t *testing.T) { ... }
func TestDataPacket_StructPayload_ComplexJSON(t *testing.T) { ... }

Affected area

• Proto definitions
• Generated bindings

Compatibility / migration

Test-only. No behavior change.

Additional context

Identified during QA review. These tests are particularly valuable before proto schema changes — they serve as the safety net that buf-breaking checks complement.