fix: handle missing span description and non-dict MSTeams error JSON

[billyvg]

Fixes two bugs surfaced by Sentry alert monitoring.

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Fix 1: KeyError 'description' in N+1 DB span detector (SENTRY-5QVD — HIGH actionability)

Issue: SENTRY-5QVD — 291K+ occurrences since 2026-06-25.

Root cause: mn_plus_one_db_span_detector.py:258 did a direct dict lookup db_span["description"]. Spans arriving from the process-segments consumer can omit the "description" field, causing a KeyError. This exception is caught by the outer try/except in detect_performance_problems() which logs "Failed to detect performance problems" and suppresses detection entirely for affected spans.

Evidence from event stacktrace:

File "sentry/issue_detection/detectors/mn_plus_one_db_span_detector.py", line 258, in _maybe_performance_problem
    desc=db_span["description"],
KeyError: 'description'

Local variable common_parent_span showed "downsampled_retention_days":"30" and no "description" key.

Fix: Use db_span.get("description", "") — matches the pattern used in evidence_data throughout the same function.

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Fix 2: AttributeError on non-dict MSTeams error JSON (SENTRY-5PKQ — MEDIUM)

Issue: SENTRY-5PKQ — 2573 occurrences, 240 users affected since 2026-05-11.

Root cause: msteams/metrics.py:34 called error.json.get(...) without verifying error.json is a dict. When Microsoft's OAuth endpoint returns a 401 with certain error payloads (e.g. AADSTS7000222 — expired client secret), error.json is a string, not a dict. .get() on a string raises AttributeError, which bubbles up as a noisy secondary exception obscuring the real ApiUnauthorized error.

Evidence from event stacktrace:

File "sentry/integrations/msteams/metrics.py", line 34, in translate_msteams_api_error
    if error.json.get("error", {}).get("code") in MSTEAMS_HALT_ERROR_CODES:
AttributeError: 'str' object has no attribute 'get'

The underlying cause was an expired Azure app client secret (the primary operator issue), but the secondary AttributeError was generating an additional 2500+ Sentry events.

Fix: Use isinstance(error.json, dict) instead of a truthiness check, so non-dict payloads fall through to the existing IntegrationError branch cleanly.

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