fix(go/plugins/googlegenai): include thoughtSignature in part metadata

go/plugins/googlegenai/gemini.go

@@ -807,6 +807,7 @@ func translateCandidate(cand *genai.Candidate) (*ai.ModelResponse, error) {
 		if part.FileData != nil {
 			partFound++
 			p = ai.NewMediaPart(part.FileData.MIMEType, part.FileData.FileURI)
+
 		}
 		if part.FunctionCall != nil {
 			partFound++
@@ -836,6 +837,13 @@ func translateCandidate(cand *genai.Candidate) (*ai.ModelResponse, error) {
 			continue
 		}
 
+                if len(part.ThoughtSignature) > 0 {
+                        if p.Metadata == nil {
+                                p.Metadata = make(map[string]any)
+                        }
+                        p.Metadata["signature"] = part.ThoughtSignature
+                }
+
 		msg.Content = append(msg.Content, p)
 	}
 	m.Message = msg
@@ -892,37 +900,33 @@ func toGeminiParts(parts []*ai.Part) ([]*genai.Part, error) {
 
 // toGeminiPart converts a [ai.Part] to a [genai.Part].
 func toGeminiPart(p *ai.Part) (*genai.Part, error) {
+	var gp *genai.Part
 	switch {
 	case p.IsReasoning():
-		// TODO: go-genai does not support genai.NewPartFromThought()
-		signature := []byte{}
-		if p.Metadata != nil {
-			if sig, ok := p.Metadata["signature"].([]byte); ok {
-				signature = sig
-			}
+		// NOTE: go-genai does not support genai.NewPartFromThought()
+		gp = &genai.Part{
+			Thought: true,
+			Text:    p.Text,
 		}
-		return &genai.Part{
-			Thought:          true,
-			Text:             p.Text,
-			ThoughtSignature: signature,
-		}, nil
 	case p.IsText():
-		return genai.NewPartFromText(p.Text), nil
+		gp = genai.NewPartFromText(p.Text)
 	case p.IsMedia():
 		if strings.HasPrefix(p.Text, "data:") {
 			contentType, data, err := uri.Data(p)
 			if err != nil {
 				return nil, err
 			}
-			return genai.NewPartFromBytes(data, contentType), nil
+			gp = genai.NewPartFromBytes(data, contentType)
+		} else {
+			gp = genai.NewPartFromURI(p.Text, p.ContentType)
 		}
-		return genai.NewPartFromURI(p.Text, p.ContentType), nil
 	case p.IsData():
 		contentType, data, err := uri.Data(p)
 		if err != nil {
 			return nil, err
 		}
-		return genai.NewPartFromBytes(data, contentType), nil
+		gp = genai.NewPartFromBytes(data, contentType)
+
 	case p.IsToolResponse():
 		toolResp := p.ToolResponse
 		var output map[string]any
@@ -934,8 +938,7 @@ func toGeminiPart(p *ai.Part) (*genai.Part, error) {
 				"content": toolResp.Output,
 			}
 		}
-		fr := genai.NewPartFromFunctionResponse(toolResp.Name, output)
-		return fr, nil
+		gp = genai.NewPartFromFunctionResponse(toolResp.Name, output)
 	case p.IsToolRequest():
 		toolReq := p.ToolRequest
 		var input map[string]any
@@ -946,11 +949,19 @@ func toGeminiPart(p *ai.Part) (*genai.Part, error) {
 				"input": toolReq.Input,
 			}
 		}
-		fc := genai.NewPartFromFunctionCall(toolReq.Name, input)
-		return fc, nil
+		gp = genai.NewPartFromFunctionCall(toolReq.Name, input)
+
 	default:
-		panic("unknown part type in a request")
+		return nil, fmt.Errorf("unknown part in the request: %q", p.Kind)
 	}
+
+	if p.Metadata != nil {
+		if sig, ok := p.Metadata["signature"].([]byte); ok {
+			gp.ThoughtSignature = sig
+		}
+	}
+
+	return gp, nil
 }
 
 // validToolName checks whether the provided tool name matches the