test: Fix test pollution and determinism for shuffle test runs

[joshuarli]

Background

As part of investigating test ordering sensitivity with a randomised xdist shuffle workflow (SENTRY_SHUFFLE_TESTS=1), we identified 20+ tests that fail consistently in shuffled order but pass 5/5 in isolation. This PR collects all resulting fixes and skip annotations across 59 test files.

Note: fix/testutils-sdk-reset-trace should merge before this one (a few tests here import reset_trace_context).

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Changes by category

Pollution skips

Tests confirmed to pass in isolation (5/5) but fail in shuffled order due to shared state from a prior test. Each is annotated with the root cause in its reason= string. Full list with diagnoses in .agents/skills/fix-flaky-tests/references/skipped-pollution-tests.md.

Key skips:

Test	Root cause
test_no_work_is_no_op (hybrid_cloud)	tombstone/outbox rows left by prior test
test_webhook_request_saved (sentry_apps)	Redis buffer contaminated by prior test
test_top_events_with_metrics_enhanced_..._has_filter_falls_back_to_indexed_data (MEP)	Snuba transaction data from prior test
test_impersonation_enforces_rate_limits_when_disabled (ratelimit)	stale Redis rate-limit counter
test_concurrent_request_rate_limiting (ratelimit)	stale concurrent counter from test_request_finishes
test_node_lambda_setup_layer_success (aws_lambda)	prior test leaves Lambda integration state
test_basic (reprocessing2)	Snuba event data from prior test in query results
objectstore classes	live_server socket leak (fd checker)
spans cluster variants	Redis Cluster shared across xdist workers
+ 10 more	see linked reference doc

Ordering fixes

Tests that asserted exact list ordering from unordered querysets:

• test_discover_saved_queries.py::test_post — sorted() on project list comparison
• test_base_data_condition_group.py::test_update and test_update_trigger__valid_logic_type — .order_by("id") on conditions queryset

Timing and propagation fixes

• Retry loops for Snuba/ClickHouse propagation in test_event_manager.py
• optimize_snuba_table() calls before assertions in test_reprocessing2.py and test_unmerge.py
• Larger batch_size in test_unmerge.py to avoid inter-batch Snuba races

Snowflake ID exhaustion fixes

• test_tasks.py (dynamic_sampling), test_merge.py: wrap setUp/test with time_machine.travel(tick=True) so the clock advances and Snowflake IDs don't collide under freeze_time

Race condition fixes

• test_outbox.py: restore threading.Barrier for not_flush_all__upper_bound to prevent barrier timeout
• test_handler.py: use reset_trace_context() to clear the inherited span inside isolation_scope()

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Test plan

All listed tests pass 5/5 in isolation. The overall shuffle suite went from ~15% failure rate to <2% (residual xdist hang infrastructure issues) over 40 consecutive 16-shard runs.