Fix GrpcGenerator type mapping for commands and nullable primitives

- Add proper complex type mapping for command results (same as queries already had)
- Handle nullable primitives (long?, int?, etc.) with default value fallback
- Fixes CS0029 and CS0266 compilation errors in generated gRPC service implementations

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Svrnty.CQRS.Grpc.Generators/ProtoTypeMapper.cs

@@ -108,6 +108,12 @@ internal static class ProtoFileTypeMapper
                 needsImport = true;
                 importPath = "google/protobuf/timestamp.proto";
                 return "google.protobuf.Timestamp";
+            case "DateOnly":
+                // DateOnly serialized as string (YYYY-MM-DD format)
+                return "string";
+            case "TimeOnly":
+                // TimeOnly serialized as string (HH:mm:ss format)
+                return "string";
             case "TimeSpan":
                 needsImport = true;
                 importPath = "google/protobuf/duration.proto";