test: the request commit check sees Core DML and row locks

A db.execute(update/delete) fires no flush and stages nothing, so a handler whose
only write was Core DML could forget its commit and stay green. The check now
asks Postgres whether the request opened a writing transaction. It found one
real case: a repeated issue send returned with the session row still locked.

backend/tests/conftest.py

@@ -105,6 +105,11 @@ def pytest_configure(config):
         )
 
 
+def _write_xact(db) -> str | None:
+    """The id of the open transaction if it has written anything, else None (a read never takes one)."""
+    return db.scalar(text("SELECT pg_current_xact_id_if_assigned()::text"))
+
+
 def _request_dep(db, unfinished: list[str]):
     """`get_db` for a test request: the test's own session, plus the check a shared session hides.
 
@@ -124,6 +129,7 @@ def _request_dep(db, unfinished: list[str]):
     def dep(conn: HTTPConnection):
         db.flush()
         state = {"dirty": False}
+        before = _write_xact(db)
 
         def flushed(session, ctx):
             state["dirty"] = True
@@ -138,7 +144,12 @@ def _request_dep(db, unfinished: list[str]):
         try:
             yield db
             staged = db.new or db.deleted or any(db.is_modified(o) for o in db.dirty)
-            if state["dirty"] or staged:
+            # Core DML (`db.execute(update(...))`) fires no flush and stages nothing; the database
+            # itself says whether a write is open — a transaction that wrote and was not committed
+            # holds an id it did not hold when the request began (see `release_read_connection`). A
+            # row lock (`FOR UPDATE`) takes one too: the handler ends what it locked.
+            core_dml = (after := _write_xact(db)) is not None and after != before
+            if state["dirty"] or staged or core_dml:
                 unfinished.append(f"{conn.scope.get('method', 'WS')} {conn.url.path}")
         except Exception:
             db.rollback()