fix: handle missing description key in MN+1 DB span detector

[billyvg]

Summary

Fixes a KeyError: 'description' crash in the MN+1 DB span detector that has been firing 333K+ times since 2026-06-25, affecting the process-segments consumer in production.

Root cause: ContinuingMNPlusOne._maybe_performance_problem() accessed db_span["description"] directly. Spans processed by the standalone segment pipeline (spans.consumers.process_segments) may not include a description field, causing an unhandled KeyError that swallowed the entire performance detection step via a broad except in detect_performance_problems.

Fix: Replace db_span["description"] with db_span.get("description", "") in both places in _maybe_performance_problem() — once for the PerformanceProblem.desc field and once for the evidence_display notification body.

Evidence

• Sentry issue: SENTRY-5QVD — 333,450 occurrences, first seen 2026-06-25, ongoing
• Seer actionability: high
• Stack trace confirms the crash is at mn_plus_one_db_span_detector.py:258 in _maybe_performance_problem
• Seer analysis: "Use .get('description', '') to safely access the db span description"

Changes

• src/sentry/issue_detection/detectors/mn_plus_one_db_span_detector.py: Use .get("description", "") instead of direct key access in two places
• tests/sentry/issue_detection/test_m_n_plus_one_db_detector.py: Add test_does_not_raise_when_db_span_missing_description — verifies the detector returns a problem with desc="" instead of crashing

Claude Session

https://claude.ai/code/session_01KBB4d6THmfZynYkkcPNxTh

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

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Generated by Claude Code

[github-actions]

Backend Test Failures

Failures on 9854e84 in this run:

tests/sentry/issue_detection/test_m_n_plus_one_db_detector.py::MNPlusOneDBDetectorTest::test_does_not_raise_when_db_span_missing_description — log

[gw1] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/issue_detection/test_m_n_plus_one_db_detector.py:317: in test_does_not_raise_when_db_span_missing_description
    assert len(problems) == 1
E   assert 0 == 1
E    +  where 0 = len([])