Fix output repair diagnostics and concurrency tests

internal/usecase/repairer_test.go

@@ -234,6 +234,23 @@ func TestFormatRepairDiagnosticsBoundsAndPreservesData(t *testing.T) {
 				}
 			},
 		},
+		{
+			name: "no room for partial diagnostic",
+			errors: func() []string {
+				omission, _ := json.Marshal(omittedRepairDiagnostics)
+				return []string{
+					strings.Repeat("x", maxRepairDiagnosticBytes-len(omission)-5),
+					strings.Repeat("y", 128),
+				}
+			}(),
+			wantOmission: true,
+			check: func(t *testing.T, got []string) {
+				t.Helper()
+				if len(got) != 2 || got[1] != omittedRepairDiagnostics {
+					t.Fatalf("diagnostics = %#v", got)
+				}
+			},
+		},
 		{
 			name:   "invalid UTF-8",
 			errors: []string{"broken\xffinput"},