feat: use appearance.Fetcher in agentapi (#11770)

This PR updates the Agent API to use the appearance.Fetcher, which is set by entitlement code in Enterprise coderd.

This brings the agentapi into compliance with the Enterprise feature.

coderd/agentapi/servicebanner_internal_test.go

@@ -0,0 +1,72 @@
+package agentapi
+
+import (
+	"context"
+	"sync/atomic"
+	"testing"
+
+	"golang.org/x/xerrors"
+
+	agentproto "github.com/coder/coder/v2/agent/proto"
+	"github.com/coder/coder/v2/coderd/appearance"
+	"github.com/coder/coder/v2/codersdk"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGetServiceBanner(t *testing.T) {
+	t.Parallel()
+
+	t.Run("OK", func(t *testing.T) {
+		t.Parallel()
+
+		cfg := codersdk.ServiceBannerConfig{
+			Enabled:         true,
+			Message:         "hello world",
+			BackgroundColor: "#000000",
+		}
+
+		var ff appearance.Fetcher = fakeFetcher{cfg: codersdk.AppearanceConfig{ServiceBanner: cfg}}
+		ptr := atomic.Pointer[appearance.Fetcher]{}
+		ptr.Store(&ff)
+
+		api := &ServiceBannerAPI{
+			appearanceFetcher: &ptr,
+		}
+
+		resp, err := api.GetServiceBanner(context.Background(), &agentproto.GetServiceBannerRequest{})
+		require.NoError(t, err)
+
+		require.Equal(t, &agentproto.ServiceBanner{
+			Enabled:         cfg.Enabled,
+			Message:         cfg.Message,
+			BackgroundColor: cfg.BackgroundColor,
+		}, resp)
+	})
+
+	t.Run("FetchError", func(t *testing.T) {
+		t.Parallel()
+
+		expectedErr := xerrors.New("badness")
+		var ff appearance.Fetcher = fakeFetcher{err: expectedErr}
+		ptr := atomic.Pointer[appearance.Fetcher]{}
+		ptr.Store(&ff)
+
+		api := &ServiceBannerAPI{
+			appearanceFetcher: &ptr,
+		}
+
+		resp, err := api.GetServiceBanner(context.Background(), &agentproto.GetServiceBannerRequest{})
+		require.Error(t, err)
+		require.ErrorIs(t, err, expectedErr)
+		require.Nil(t, resp)
+	})
+}
+
+type fakeFetcher struct {
+	cfg codersdk.AppearanceConfig
+	err error
+}
+
+func (f fakeFetcher) Fetch(context.Context) (codersdk.AppearanceConfig, error) {
+	return f.cfg, f.err
+}