runBackup rejects a contentConcurrency that is not a positive integer

A library object whose contentConcurrency was missing, 0 or negative
started no worker, so the backup stored no original, dropped the earlier
entries of failures.json and reported no error. runBackup now throws a
RangeError for any limit that is not a positive integer, as BoundedPool
does, before it takes the lock.

Model: opus-5-5

src/backup.ts

@@ -933,6 +933,14 @@ export const runBackup = async (
                 "open the library with one)",
         );
     }
+    // As `BoundedPool` does. A limit below 1, or one that is not a number,
+    // would back up no original and end with no error.
+    const { contentConcurrency } = lib;
+    if (!Number.isInteger(contentConcurrency) || contentConcurrency < 1) {
+        throw new RangeError(
+            `contentConcurrency must be a positive integer, got ${contentConcurrency}`,
+        );
+    }
     if (opts.lockHeld) {
         return runLockedBackup(lib, opts, downloadDirectory);
     }