feat(server): add Tool call error categories

internal/util/errors.go

@@ -0,0 +1,42 @@
+// Copyright 2025 Google LLC
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package util
+
+import "fmt"
+
+type ErrorCategory string
+
+const (
+	CategoryAgent  ErrorCategory = "AGENT_ERROR"
+	CategoryServer ErrorCategory = "SERVER_ERROR"
+)
+
+// ToolboxError is the interface all custom errors must satisfy
+type ToolboxError interface {
+	error
+}
+
+// Agent Errors
+type AgentError struct{ Msg string }
+
+func (e *AgentError) Error() string           { return e.Msg }
+func (e *AgentError) Category() ErrorCategory { return CategoryAgent }
+
+// Server Errors
+type ServerError struct {
+	Msg   string
+	Cause error
+}
+
+func (e *ServerError) Error() string           { return fmt.Sprintf("%s: %v", e.Msg, e.Cause) }
+func (e *ServerError) Category() ErrorCategory { return CategoryServer }