test(epp): add unit tests for error package (#1052)

* test(epp): add unit tests for error package

- Add comprehensive tests for Error.Error() method
- Add tests for CanonicalCode() function with edge cases
- Test all error code constants
- Achieve 100% test coverage for pkg/epp/util/error

Signed-off-by: Naoki Sega <nsegaster@gmail.com>

* test(epp): refactor TestErrorConstants to use single map

---------

Signed-off-by: Naoki Sega <nsegaster@gmail.com>

Author: nsega

pkg/epp/util/error/error_test.go

@@ -0,0 +1,218 @@
+/*
+Copyright 2025 The Kubernetes Authors.
+
+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 error
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestError_Error(t *testing.T) {
+	tests := []struct {
+		name string
+		err  Error
+		want string
+	}{
+		{
+			name: "BadRequest error",
+			err: Error{
+				Code: BadRequest,
+				Msg:  "invalid model name",
+			},
+			want: "inference gateway: BadRequest - invalid model name",
+		},
+		{
+			name: "Internal error",
+			err: Error{
+				Code: Internal,
+				Msg:  "unexpected condition",
+			},
+			want: "inference gateway: Internal - unexpected condition",
+		},
+		{
+			name: "ModelServerError",
+			err: Error{
+				Code: ModelServerError,
+				Msg:  "connection timeout",
+			},
+			want: "inference gateway: ModelServerError - connection timeout",
+		},
+		{
+			name: "BadConfiguration error",
+			err: Error{
+				Code: BadConfiguration,
+				Msg:  "missing required field",
+			},
+			want: "inference gateway: BadConfiguration - missing required field",
+		},
+		{
+			name: "InferencePoolResourceExhausted error",
+			err: Error{
+				Code: InferencePoolResourceExhausted,
+				Msg:  "no available pods",
+			},
+			want: "inference gateway: InferencePoolResourceExhausted - no available pods",
+		},
+		{
+			name: "Unknown error",
+			err: Error{
+				Code: Unknown,
+				Msg:  "something went wrong",
+			},
+			want: "inference gateway: Unknown - something went wrong",
+		},
+		{
+			name: "Empty message",
+			err: Error{
+				Code: BadRequest,
+				Msg:  "",
+			},
+			want: "inference gateway: BadRequest - ",
+		},
+		{
+			name: "Empty code",
+			err: Error{
+				Code: "",
+				Msg:  "error occurred",
+			},
+			want: "inference gateway:  - error occurred",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.err.Error(); got != tt.want {
+				t.Errorf("Error.Error() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestCanonicalCode(t *testing.T) {
+	tests := []struct {
+		name string
+		err  error
+		want string
+	}{
+		{
+			name: "Error type with BadRequest code",
+			err: Error{
+				Code: BadRequest,
+				Msg:  "invalid input",
+			},
+			want: BadRequest,
+		},
+		{
+			name: "Error type with Internal code",
+			err: Error{
+				Code: Internal,
+				Msg:  "server error",
+			},
+			want: Internal,
+		},
+		{
+			name: "Error type with ModelServerError code",
+			err: Error{
+				Code: ModelServerError,
+				Msg:  "model unavailable",
+			},
+			want: ModelServerError,
+		},
+		{
+			name: "Error type with BadConfiguration code",
+			err: Error{
+				Code: BadConfiguration,
+				Msg:  "invalid config",
+			},
+			want: BadConfiguration,
+		},
+		{
+			name: "Error type with InferencePoolResourceExhausted code",
+			err: Error{
+				Code: InferencePoolResourceExhausted,
+				Msg:  "no resources",
+			},
+			want: InferencePoolResourceExhausted,
+		},
+		{
+			name: "Error type with Unknown code",
+			err: Error{
+				Code: Unknown,
+				Msg:  "unknown error",
+			},
+			want: Unknown,
+		},
+		{
+			name: "Error type with empty code",
+			err: Error{
+				Code: "",
+				Msg:  "no code provided",
+			},
+			want: "",
+		},
+		{
+			name: "Non-Error type",
+			err:  errors.New("standard go error"),
+			want: Unknown,
+		},
+		{
+			name: "Nil error",
+			err:  nil,
+			want: Unknown,
+		},
+		{
+			name: "Custom error type that is not Error",
+			err:  customError{msg: "custom error"},
+			want: Unknown,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := CanonicalCode(tt.err); got != tt.want {
+				t.Errorf("CanonicalCode() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+// customError is a helper type for testing non-Error error types
+type customError struct {
+	msg string
+}
+
+func (e customError) Error() string {
+	return e.msg
+}
+
+func TestErrorConstants(t *testing.T) {
+	// Verify that error constants match their expected string values
+	tests := map[string]string{
+		Unknown:                        "Unknown",
+		BadRequest:                     "BadRequest",
+		Internal:                       "Internal",
+		ModelServerError:               "ModelServerError",
+		BadConfiguration:               "BadConfiguration",
+		InferencePoolResourceExhausted: "InferencePoolResourceExhausted",
+	}
+
+	for constant, expected := range tests {
+		if constant != expected {
+			t.Errorf("Constant value %q != expected %q", constant, expected)
+		}
+	}
+}