test(observability): cover the telemetry success path

Adds a fake OTLP collector so NewTelemetryClient can be exercised against a
reachable endpoint: it builds all three providers and both ForceFlush and
Shutdown export and tear down without error. Previously only the
not-configured and unreachable-dial paths were covered.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/pkg/observability/otelclient_test.go

@@ -8,8 +8,37 @@ import (
 	"time"
 
 	"pentagi/pkg/config"
+
+	collogsv1 "go.opentelemetry.io/proto/otlp/collector/logs/v1"
+	colmetricsv1 "go.opentelemetry.io/proto/otlp/collector/metrics/v1"
+	coltracev1 "go.opentelemetry.io/proto/otlp/collector/trace/v1"
+	"google.golang.org/grpc"
 )
 
+type fakeLogsCollector struct {
+	collogsv1.UnimplementedLogsServiceServer
+}
+
+func (fakeLogsCollector) Export(context.Context, *collogsv1.ExportLogsServiceRequest) (*collogsv1.ExportLogsServiceResponse, error) {
+	return &collogsv1.ExportLogsServiceResponse{}, nil
+}
+
+type fakeMetricsCollector struct {
+	colmetricsv1.UnimplementedMetricsServiceServer
+}
+
+func (fakeMetricsCollector) Export(context.Context, *colmetricsv1.ExportMetricsServiceRequest) (*colmetricsv1.ExportMetricsServiceResponse, error) {
+	return &colmetricsv1.ExportMetricsServiceResponse{}, nil
+}
+
+type fakeTraceCollector struct {
+	coltracev1.UnimplementedTraceServiceServer
+}
+
+func (fakeTraceCollector) Export(context.Context, *coltracev1.ExportTraceServiceRequest) (*coltracev1.ExportTraceServiceResponse, error) {
+	return &coltracev1.ExportTraceServiceResponse{}, nil
+}
+
 func TestNewTelemetryClient_EmptyEndpointReturnsErrNotConfigured(t *testing.T) {
 	_, err := NewTelemetryClient(context.Background(), &config.Config{TelemetryEndpoint: ""})
 	if !errors.Is(err, ErrNotConfigured) {
@@ -17,6 +46,36 @@ func TestNewTelemetryClient_EmptyEndpointReturnsErrNotConfigured(t *testing.T) {
 	}
 }
 
+func TestNewTelemetryClient_SuccessPathExportsAndShutsDown(t *testing.T) {
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("listen: %v", err)
+	}
+	srv := grpc.NewServer()
+	collogsv1.RegisterLogsServiceServer(srv, fakeLogsCollector{})
+	colmetricsv1.RegisterMetricsServiceServer(srv, fakeMetricsCollector{})
+	coltracev1.RegisterTraceServiceServer(srv, fakeTraceCollector{})
+	go func() { _ = srv.Serve(ln) }()
+	defer srv.Stop()
+
+	client, err := NewTelemetryClient(context.Background(), &config.Config{TelemetryEndpoint: ln.Addr().String()})
+	if err != nil {
+		t.Fatalf("NewTelemetryClient: %v", err)
+	}
+	if client.Logger() == nil || client.Tracer() == nil || client.Meter() == nil {
+		t.Fatal("a telemetry provider is nil")
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	if err := client.ForceFlush(ctx); err != nil {
+		t.Fatalf("ForceFlush: %v", err)
+	}
+	if err := client.Shutdown(ctx); err != nil {
+		t.Fatalf("Shutdown: %v", err)
+	}
+}
+
 // H4 guard: a set-but-unreachable collector must not hang startup. The endpoint
 // accepts the TCP connection but never completes the gRPC handshake, so a
 // WithBlock dial would wait forever without the internal DefaultDialTimeout that