Omit definitions from returned references

Returning a gateway frontend result serialized the solved references'
LLB definitions again. The Return handler already resolves those
references by ID [1], so a large definition plus result metadata could
exceed the gRPC message limit after Solve succeeded.

Omit definitions from references serialized in Return. Keep them in
Solve responses and client references so Reference.ToState can still
reconstruct their LLB state.

Prepared by Codex on Tamir's behalf.

[1]: https://github.com/moby/buildkit/blob/64aefaef4d7dbb993156a0839958e4342aeb4c51/frontend/gateway/gateway.go#L1102-L1168

Signed-off-by: Tamir Duberstein <tamird@gmail.com>

Author: tamird

frontend/gateway/grpcclient/client.go

@@ -97,7 +97,8 @@ func convertRef(ref client.Reference) (*pb.Ref, error) {
 	if !ok {
 		return nil, errors.Errorf("invalid return reference type %T", ref)
 	}
-	return &pb.Ref{Id: r.id, Def: r.def}, nil
+	// Return only needs the ID of the reference held by the server.
+	return &pb.Ref{Id: r.id}, nil
 }
 
 func RunFromEnvironment(ctx context.Context, f client.BuildFunc) error {