Implement json.Unmarshaler to CreateMessageResult (#191)

When calling `ServerSession.CreateMessage`, will get an json unmarshal
error because `CreateMessageResult.Content` is an interface, and it
doesn't implement `json.Unmarshaller`.

Implement the `json.Unmarshaller` to `CreateMessageResult`, let it able
to unmarshal the client response without error

mcp/mcp_test.go

@@ -125,7 +125,7 @@ func TestEndToEnd(t *testing.T) {
 	loggingMessages := make(chan *LoggingMessageParams, 100) // big enough for all logging
 	opts := &ClientOptions{
 		CreateMessageHandler: func(context.Context, *ClientSession, *CreateMessageParams) (*CreateMessageResult, error) {
-			return &CreateMessageResult{Model: "aModel"}, nil
+			return &CreateMessageResult{Model: "aModel", Content: &TextContent{}}, nil
 		},
 		ToolListChangedHandler:     func(context.Context, *ClientSession, *ToolListChangedParams) { notificationChans["tools"] <- 0 },
 		PromptListChangedHandler:   func(context.Context, *ClientSession, *PromptListChangedParams) { notificationChans["prompts"] <- 0 },

mcp/protocol.go

@@ -264,6 +264,23 @@ type CreateMessageResult struct {
 	StopReason string `json:"stopReason,omitempty"`
 }
 
+func (r *CreateMessageResult) UnmarshalJSON(data []byte) error {
+	type result CreateMessageResult // avoid recursion
+	var wire struct {
+		result
+		Content *wireContent `json:"content"`
+	}
+	if err := json.Unmarshal(data, &wire); err != nil {
+		return err
+	}
+	var err error
+	if wire.result.Content, err = contentFromWire(wire.Content, map[string]bool{"text": true, "image": true, "audio": true}); err != nil {
+		return err
+	}
+	*r = CreateMessageResult(wire.result)
+	return nil
+}
+
 type GetPromptParams struct {
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.