Use sentinel errors for campaign validation failures

Follow the cookiebanner pattern: grouped var Err* sentinels in the
service package, wrapped with fmt.Errorf where context is needed, and
explicit errors.Is checks in GraphQL resolvers.

Signed-off-by: Cursor Agent <cursoragent@cursor.com>

Co-authored-by: Bryan FRIMIN <bryan@frimin.fr>

pkg/accessreview/errors_test.go

@@ -1,85 +0,0 @@
-// Copyright (c) 2026 Probo Inc <hello@probo.com>.
-//
-// Permission is hereby granted, free of charge, to any person obtaining a copy
-// of this software and associated documentation files (the "Software"), to deal
-// in the Software without restriction, including without limitation the rights
-// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
-// copies of the Software, and to permit persons to whom the Software is
-// furnished to do so, subject to the following conditions:
-//
-// The above copyright notice and this permission notice shall be included in
-// all copies or substantial portions of the Software.
-//
-// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-// SOFTWARE.
-
-package accessreview
-
-import (
-	"errors"
-	"fmt"
-	"testing"
-
-	"github.com/stretchr/testify/assert"
-	"go.probo.inc/probo/pkg/coredata"
-	"go.probo.inc/probo/pkg/gid"
-)
-
-func TestCampaignClientErrors(t *testing.T) {
-	t.Parallel()
-
-	statusErr := &CampaignInvalidStatusError{
-		Operation: "start",
-		Status:    coredata.AccessReviewCampaignStatusInProgress,
-		Expected:  coredata.AccessReviewCampaignStatusDraft,
-	}
-	sourceErr := &CampaignSourceOrganizationMismatchError{
-		Operation: "update",
-		SourceID:  gid.GID("source-id"),
-	}
-
-	tests := []struct {
-		name string
-		err  error
-		want bool
-	}{
-		{
-			name: "no scope sources",
-			err:  ErrCampaignNoScopeSources,
-			want: true,
-		},
-		{
-			name: "invalid status",
-			err:  statusErr,
-			want: true,
-		},
-		{
-			name: "wrapped invalid status",
-			err:  fmt.Errorf("cannot start access review campaign: %w", statusErr),
-			want: true,
-		},
-		{
-			name: "source organization mismatch",
-			err:  sourceErr,
-			want: true,
-		},
-		{
-			name: "internal error",
-			err:  errors.New("cannot lock campaign: timeout"),
-			want: false,
-		},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			t.Parallel()
-
-			assert.Equal(t, tt.want, IsCampaignClientError(tt.err))
-		})
-	}
-}