serialize_modification_request crashed toward a string 'None' instead of
JSON null for requests whose test_id was nulled by the SET NULL cascade.
Also corrects two comments that still described the debunked ordering
theory instead of the actual SET NULL fix, and fixes a test that
asserted post-cascade state without committing first (the ORM only
picks up out-of-band DB-side SET NULL on already-loaded objects after
expire_on_commit forces a re-read, matching real router behavior).
Approving a remove_test modification request deletes the underlying Test
row, which was cascading through test_id's ON DELETE CASCADE and wiping
out the request row itself before it could be read back. Changed to
ON DELETE SET NULL so the audit record (justification, reviewer,
decision) survives. Adds regression coverage plus double-approve/reject
idempotency tests.
kitos