feat: define core types and interfaces

a2a/errors.go

@@ -0,0 +1,17 @@
+package a2a
+
+import "fmt"
+
+var ErrTaskNotFound = fmt.Errorf("task not found")
+
+var ErrTaskNotCancelable = fmt.Errorf("task cannot be canceled")
+
+var ErrPushNotificationNotSupported = fmt.Errorf("push notification not supported")
+
+var ErrUnsupportedOperation = fmt.Errorf("this operation is not supported")
+
+var ErrUnsupportedContentType = fmt.Errorf("incompatible content types")
+
+var ErrInvalidAgentResponse = fmt.Errorf("invalid agent response")
+
+var ErrInvalidRequest = fmt.Errorf("invalid request")