fix(perf-detection): handle missing description field in MN+1 DB span detector

Spans processed by the segment consumer may not always include a
`description` key. Accessing `db_span["description"]` raised KeyError
causing the entire performance problem detection to fail (SENTRY-5QVD,
210K+ events). Changed both callsites to use `.get("description", "")`.

Also downgrade `logger.exception` to `logger.warning` for
`DetectorGroup.DoesNotExist` in `_get_detector_for_group` — this is an
expected condition for legacy groups that predate DetectorGroup
associations, and the code already handles it gracefully by falling
through to alternative detector lookups (SENTRY-5R4N, 24K events, 4K
users). Using `logger.exception` was generating noisy error-level
log events with full tracebacks for a known benign case.

Fixes SENTRY-5QVD
Ref SENTRY-5R4N

Author: claude

src/sentry/issue_detection/detectors/mn_plus_one_db_span_detector.py

@@ -255,7 +255,7 @@ def _maybe_performance_problem(self) -> PerformanceProblem | None:
         return PerformanceProblem(
             fingerprint=self._fingerprint(db_span["hash"], common_parent_span),
             op="db",
-            desc=db_span["description"],
+            desc=db_span.get("description", ""),
             type=PerformanceNPlusOneGroupType,
             parent_span_ids=[common_parent_span["span_id"]],
             cause_span_ids=db_span_ids,
@@ -280,7 +280,7 @@ def _maybe_performance_problem(self) -> PerformanceProblem | None:
                     name="Offending Spans",
                     value=get_notification_attachment_body(
                         "db",
-                        db_span["description"],
+                        db_span.get("description", ""),
                     ),
                     # Has to be marked important to be displayed in the notifications
                     important=True,

src/sentry/workflow_engine/processors/detector.py

@@ -152,7 +152,7 @@ def _get_detector_for_group(group: Group) -> Detector:
         if detector is not None:
             return detector
     except DetectorGroup.DoesNotExist:
-        logger.exception(
+        logger.warning(
             "DetectorGroup not found for group",
             extra={"group_id": group.id},
         )