Flaky: test_countersign checks for '004217' and 'mrn' anywhere in countersigned JSON #1813

tests/test_countersign.py::test_countersigning_leaves_the_receipt_and_its_seal_untouched searches the whole countersigned JSON for the substrings '004217' and 'mrn'. That JSON contains random base64 (TSA token, signatures), so it fails at random: one run's token contained "mrnJdemr". Fix: decode the receipt and assert on the specific fields, not substrings of the serialized JSON. Found while fixing #1510's CI (shard 1).