Manual code-review pass (code-review-python checklist) flagged that no test
exercised default_filter already restricting level (e.g. Alerts' ERROR-only
default) combined with every active group specifying its own levels. The
replace-not-AND behavior is deliberate per SIENTIAPDE-2011's design doc and
ticket text, but was previously only covered by a docstring comment. Adds
test_compute_query_time_filter_union_replaces_default_level_restriction so a
future change reintroducing AND semantics fails a test instead of passing
silently.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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