Fix output repair diagnostics and concurrency tests

internal/usecase/repairer.go

@@ -132,7 +132,8 @@ func formatRepairDiagnostics(errors []string) string {
 		if len(encoded) > 1 {
 			separator = 1
 		}
-		if len(encoded)+separator+len(entry)+1+len(omission)+1 <= maxRepairDiagnosticBytes {
+		available := maxRepairDiagnosticBytes - len(encoded) - separator - 1 - len(omission) - 1
+		if len(entry) <= available {
 			if separator != 0 {
 				encoded = append(encoded, ',')
 			}
@@ -140,7 +141,9 @@ func formatRepairDiagnostics(errors []string) string {
 			continue
 		}
 
-		available := maxRepairDiagnosticBytes - len(encoded) - separator - 1 - len(omission) - 1
+		if available < len(`""`) {
+			break
+		}
 		if separator != 0 {
 			encoded = append(encoded, ',')
 		}
@@ -157,17 +160,24 @@ func formatRepairDiagnostics(errors []string) string {
 
 func truncateDiagnosticJSONValue(value string, maxBytes int) []byte {
 	if maxBytes < len(`""`) {
-		return []byte(`""`)
+		return nil
 	}
 
-	low, high := 0, len(value)
+	boundaries := []int{0}
+	for end := 0; end < len(value); {
+		_, size := utf8.DecodeRuneInString(value[end:])
+		end += size
+		if end+len(`""`) > maxBytes {
+			break
+		}
+		boundaries = append(boundaries, end)
+	}
+
+	low, high := 0, len(boundaries)-1
 	best := []byte(`""`)
 	for low <= high {
 		mid := low + (high-low)/2
-		for mid > 0 && mid < len(value) && !utf8.RuneStart(value[mid]) {
-			mid--
-		}
-		candidate, _ := json.Marshal(value[:mid])
+		candidate, _ := json.Marshal(value[:boundaries[mid]])
 		if len(candidate) <= maxBytes {
 			best = candidate
 			low = mid + 1