Respect JSON tags when marshaling nested structs with dynamorm:"json"

Fixed marshalComplexValue to parse and use json struct tags when
marshaling nested structs to DynamoDB AttributeValues. Previously,
it used Go field names (PascalCase like DID, IsInstructionallyFunded)
which caused inconsistent casing in stored data.

Now properly extracts field names from json tags, handling both
simple tags ("fieldname") and tags with options ("fieldname,omitempty").

This ensures that nested structs with dynamorm:"json" respect the
json tag naming conventions (camelCase) when stored in DynamoDB.

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

Co-Authored-By: Claude <[email protected]>

Author: JeffersonSchuler

pkg/marshal/marshaler.go

@@ -426,8 +426,26 @@ func (m *Marshaler) marshalComplexValue(v reflect.Value) (types.AttributeValue,
 				return nil, fmt.Errorf("struct field %s: %w", field.Name, err)
 			}
 
-			// Use field name as key (could be enhanced with tag parsing)
-			structMap[field.Name] = av
+			// Parse JSON tag to get the field name
+			fieldName := field.Name
+			if jsonTag := field.Tag.Get("json"); jsonTag != "" && jsonTag != "-" {
+				// Handle json tag with options like "fieldname,omitempty"
+				if commaIdx := 0; commaIdx < len(jsonTag) {
+					for j, c := range jsonTag {
+						if c == ',' {
+							commaIdx = j
+							break
+						}
+					}
+					if commaIdx > 0 {
+						fieldName = jsonTag[:commaIdx]
+					} else if jsonTag != "" {
+						fieldName = jsonTag
+					}
+				}
+			}
+
+			structMap[fieldName] = av
 		}
 		return &types.AttributeValueMemberM{Value: structMap}, nil