Fix Go transform JSON serialization: add json tags for content field

The systemMessageTransformRequest and systemMessageTransformResponse
used anonymous structs without json tags, causing Content to serialize
as uppercase 'Content' instead of lowercase 'content'. The CLI expects
lowercase, so transform results were silently ignored.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

go/session.go

@@ -458,23 +458,27 @@ func (s *Session) registerTransformCallbacks(callbacks map[string]SectionTransfo
 	s.transformCallbacks = callbacks
 }
 
+type systemMessageTransformSection struct {
+	Content string `json:"content"`
+}
+
 type systemMessageTransformRequest struct {
-	SessionID string                              `json:"sessionId"`
-	Sections  map[string]struct{ Content string } `json:"sections"`
+	SessionID string                                   `json:"sessionId"`
+	Sections  map[string]systemMessageTransformSection `json:"sections"`
 }
 
 type systemMessageTransformResponse struct {
-	Sections map[string]struct{ Content string } `json:"sections"`
+	Sections map[string]systemMessageTransformSection `json:"sections"`
 }
 
 // handleSystemMessageTransform handles a system message transform request from the Copilot CLI.
 // This is an internal method called by the SDK when the CLI requests section transforms.
-func (s *Session) handleSystemMessageTransform(sections map[string]struct{ Content string }) (systemMessageTransformResponse, error) {
+func (s *Session) handleSystemMessageTransform(sections map[string]systemMessageTransformSection) (systemMessageTransformResponse, error) {
 	s.transformMu.Lock()
 	callbacks := s.transformCallbacks
 	s.transformMu.Unlock()
 
-	result := make(map[string]struct{ Content string })
+	result := make(map[string]systemMessageTransformSection)
 	for sectionID, data := range sections {
 		var callback SectionTransformFn
 		if callbacks != nil {
@@ -483,12 +487,12 @@ func (s *Session) handleSystemMessageTransform(sections map[string]struct{ Conte
 		if callback != nil {
 			transformed, err := callback(data.Content)
 			if err != nil {
-				result[sectionID] = struct{ Content string }{Content: data.Content}
+				result[sectionID] = systemMessageTransformSection{Content: data.Content}
 			} else {
-				result[sectionID] = struct{ Content string }{Content: transformed}
+				result[sectionID] = systemMessageTransformSection{Content: transformed}
 			}
 		} else {
-			result[sectionID] = struct{ Content string }{Content: data.Content}
+			result[sectionID] = systemMessageTransformSection{Content: data.Content}
 		}
 	}
 	return systemMessageTransformResponse{Sections: result}, nil