Fix: JsonObject fields should not use replace() method with tristate_optionals

[devunt]

Description

This PR fixes an issue where JsonObject fields incorrectly generate calls to .replace() method when tristate_optionals is enabled.

Problem

When using Ferry with JsonObject type (from built_value package) and tristate_optionals: true option enabled, the generated serializer code contains calls to .replace() method on JsonObject? fields. However, JsonObject doesn't have a replace method, causing compilation errors.

Solution

Added special handling for JsonObject type in the tristate optionals code generator to ensure it's treated as a simple value type (using direct assignment) rather than a built collection type (which would use .replace() method).

Changes

• Modified /codegen/gql_code_builder/lib/src/tristate_optionals.dart to detect JsonObject type and exclude it from being treated as a builder type

Testing

Tested with a real-world Flutter project using:

• tristate_optionals: true
• JsonObject type override for JSON scalar
• Verified that generated code now compiles successfully

Related Issues

• Fixes Error: The method 'replace' isn't defined for type 'JsonObject?' when tristate_optionals is enabled ferry#649

Before/After

Before (causes compilation error):

case 'params':
  var _$fieldValue = serializers.deserialize(value,
      specifiedType: const FullType(_i1.JsonObject)) as _i1.JsonObject;
  builder.params.replace(_$fieldValue);  // ❌ Error: replace() not defined
  break;

After (works correctly):

case 'params':
  var _$fieldValue = serializers.deserialize(value,
      specifiedType: const FullType(_i1.JsonObject)) as _i1.JsonObject;
  builder.params = _$fieldValue;  // ✅ Direct assignment
  break;