fix(norms): templates grade by the cutoffs when handed no grade; the parts test reads inside transactions (#5742)

CI 9128: the host vitals and monitor templates required the new grades, so
callers (and tests) that render them without one failed; they now fall back
to status_class. The new integration test read outside begin(), which
autobegan the session.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/integration/test_anomalies.py

@@ -360,15 +360,19 @@ def test_a_metric_learned_without_its_parts_ignores_them(app, src):
                 s.add_all(rows)
             async with s.begin():
                 await rebuild_norms(s, NOW)
-            learned = await norms_at(s, [(src, host, METRIC), (src, part, METRIC)], NOW.replace(hour=14))
+            async with s.begin():
+                learned = await norms_at(s, [(src, host, METRIC), (src, part, METRIC)],
+                                         NOW.replace(hour=14))
             async with s.begin():
                 await _report(s, src, host, at, 90.0)
                 await _report(s, src, part, at, 90.0)
-            levels = await levels_now(s, at)
+            async with s.begin():
+                levels = await levels_now(s, at)
             async with s.begin():
                 await detect(s, at)
-            opened = list((await s.execute(select(MetricAnomaly.resource_name).where(
-                MetricAnomaly.source_module == src))).scalars())
+            async with s.begin():
+                opened = list((await s.execute(select(MetricAnomaly.resource_name).where(
+                    MetricAnomaly.source_module == src))).scalars())
             return learned, levels, opened
 
     learned, levels, opened = asyncio.run(_go())