Add internal capacity error identity

internal/usecase/runner_test.go

@@ -1357,14 +1357,14 @@ func TestRunnerAdmissionUsesResolvedBackendIdentity(t *testing.T) {
 
 func TestRunnerAdmissionFailureSkipsCompletionCollaborators(t *testing.T) {
 	tests := []struct {
-		name               string
-		admissionError     error
-		wantBackendContext bool
+		name             string
+		admissionError   error
+		wantCapacityType bool
 	}{
 		{
-			name:               "capacity exhausted",
-			admissionError:     capacity.ErrCapacityExceeded,
-			wantBackendContext: true,
+			name:             "capacity exhausted",
+			admissionError:   capacity.ErrCapacityExceeded,
+			wantCapacityType: true,
 		},
 		{
 			name:           "context canceled",
@@ -1414,8 +1414,16 @@ func TestRunnerAdmissionFailureSkipsCompletionCollaborators(t *testing.T) {
 			if errors.Is(err, ErrInvalidRequest) || errors.Is(err, ErrLLMGenerate) {
 				t.Fatalf("admission error was recategorized: %v", err)
 			}
-			if tc.wantBackendContext && !strings.Contains(err.Error(), "custom") {
-				t.Fatalf("capacity error lacks backend context: %v", err)
+			var capacityErr *CapacityError
+			if tc.wantCapacityType {
+				if !errors.As(err, &capacityErr) {
+					t.Fatalf("capacity error=%v, want internal typed identity", err)
+				}
+				if capacityErr.BackendID != "custom" {
+					t.Fatalf("capacity backend ID=%q, want custom", capacityErr.BackendID)
+				}
+			} else if errors.As(err, &capacityErr) {
+				t.Fatalf("non-capacity admission error exposed typed capacity identity: %v", err)
 			}
 			if !reflect.DeepEqual(admitter.backendIDs, []string{"custom"}) {
 				t.Fatalf("admitted backend IDs=%#v, want custom", admitter.backendIDs)