gRPC: translate errors to context.Canceled (sourcegraph#47986)

The search pipeline has special treatment for `context.Canceled` errors.
However, the gRPC error returned when a context canceled does _not_
respond to `errors.Is(err, context.Canceled)`. This means that our
special treatment fails, and the error gets returned, causing the test
to fail.

A real fix here will be to add interceptors to our gRPC clients that
translates these automatically.

## Test plan

Running backend integration tests that were previously failing
consistently.

internal/gitserver/client.go

@@ -551,7 +551,9 @@ func (c *RemoteGitCommand) sendExec(ctx context.Context) (_ io.ReadCloser, errRe
  }
  r := streamio.NewReader(func() ([]byte, error) {
  msg, err := stream.Recv()
- if err != nil {
+ if status.Code(err) == codes.Canceled {
+ return nil, context.Canceled
+ } else if err != nil {
  return nil, err
  }
  return msg.GetData(), nil

internal/search/searcher/client_grpc.go

@@ -9,6 +9,8 @@ import (
  "time"
 
  "google.golang.org/grpc"
+ "google.golang.org/grpc/codes"
+ "google.golang.org/grpc/status"
 
  "github.com/sourcegraph/sourcegraph/cmd/searcher/protocol"
  "github.com/sourcegraph/sourcegraph/internal/api"
@@ -98,6 +100,8 @@ func SearchGRPC(
  msg, err := resp.Recv()
  if errors.Is(err, io.EOF) {
  return false, nil
+ } else if status.Code(err) == codes.Canceled {
+ return false, context.Canceled
  } else if err != nil {
  return false, err
  }