fix(performance-detection): handle missing 'description' key in MN+1 DB span detector

[billyvg]

Fixes a KeyError: 'description' crash (SENTRY-5QVD) that occurs in the MN+1 DB span detector when a DB span is missing the description field.

Problem

In mn_plus_one_db_span_detector.py, the _maybe_performance_problem method accessed db_span["description"] in two places (lines 258 and 283), raising a KeyError whenever a span arrives without that field. This was generating 212k+ error events in production (sentry region us, consumer process-segments).

Seer's analysis confirmed: "Use .get('description', '') to safely access the db span description."

Fix

Replace db_span["description"] with db_span.get("description", "") in both call sites:

• desc= parameter of PerformanceProblem()
• get_notification_attachment_body() call in evidence_display

This is safe because an empty string is the appropriate fallback for a missing description — no performance problem is skipped and no fingerprint logic is affected (the fingerprint uses db_span["hash"], not description).

Evidence

• Sentry issue: https://sentry.sentry.io/issues/SENTRY-5QVD
• 212,555 occurrences since 2026-06-25, last seen moments ago
• Seer actionability: high
• Stack trace points directly to these two lines
• Triggered by this Claude Code session: https://claude.ai/code/session_01RgDgeGVvxBPx4emrgy1t4w

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