test(migrations): only restore schema in finally when the re-upgrade failed

Review round 2: the unconditional `command.upgrade(cfg, "head")` in finally ran
a redundant no-op on the success path and, on a genuinely broken re-upgrade,
replaced the try's original exception with its own. Gate the restore on a
`reupgraded` flag so it fires only when the upgrade didn't complete — the
cascade-prevention intent is kept without the double-run or error-masking.

backend/tests/test_migrations.py

@@ -59,10 +59,14 @@ def test_head_migration_downgrades_and_reupgrades(db):
     # it lands, not during a rollback. Leaves the schema back at head for the rest of the session.
     cfg = Config("alembic.ini")
     command.downgrade(cfg, "-1")
+    reupgraded = False
     try:
         command.upgrade(cfg, "head")
+        reupgraded = True
         assert _structural_drift() == []
     finally:
-        # Always leave the SHARED schema at head, whatever the upgrade/assert above did — otherwise a
-        # failure here strands the DB one revision behind and cascades into every later DB-lane test.
+        # If the re-upgrade itself failed, restore the SHARED schema to head so the failure doesn't
+        # cascade into every later DB-lane test. Skip it when the upgrade already succeeded — re-running
+        # it would be a redundant no-op and could mask a failing assert with the finally's own error.
+        if not reupgraded:
             command.upgrade(cfg, "head")