test(session): cover v2 transport failure paths

Exclude generated protocol stubs from patch coverage while exercising the
reconnect, heartbeat, endpoint, and error-classification behavior directly.

Co-Authored-By: Codex <noreply@openai.com>

Author: cardyok

codecov.yml

@@ -1,2 +1,4 @@
 ignore:
   - "e2e"
+  - "pkg/session/v2/session.pb.go"
+  - "pkg/session/v2/session_grpc.pb.go"

pkg/gpud-manager/systemd/systemd_test.go

@@ -89,6 +89,12 @@ func TestWriteEnvFileRejectsInvalidSessionProtocol(t *testing.T) {
 	require.ErrorIs(t, statErr, os.ErrNotExist)
 }
 
+func TestWriteEnvFileRejectsMultipleSessionProtocols(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "gpud.env")
+	err := writeEnvFile(path, "", "", false, "v1", "v2")
+	require.ErrorContains(t, err, "expected at most one session protocol")
+}
+
 func TestProcessEnvFileLines(t *testing.T) {
 	t.Run("file without FLAGS", func(t *testing.T) {
 		// Create a temporary directory for testing

pkg/session/protocol_test.go

@@ -3,7 +3,10 @@
 
 package session
 
-import "testing"
+import (
+	"context"
+	"testing"
+)
 
 func TestParseProtocol(t *testing.T) {
 	tests := []struct {
@@ -29,3 +32,17 @@ func TestParseProtocol(t *testing.T) {
 		})
 	}
 }
+
+func TestWithProtocol(t *testing.T) {
+	op := &Op{}
+	WithProtocol("v2")(op)
+	if op.protocol != ProtocolV2 {
+		t.Fatalf("protocol = %q, want %q", op.protocol, ProtocolV2)
+	}
+}
+
+func TestNewSessionRejectsInvalidProtocol(t *testing.T) {
+	if _, err := NewSession(context.Background(), "", "", "", WithProtocol("future")); err == nil {
+		t.Fatal("NewSession accepted an unsupported protocol")
+	}
+}