feat(seer): Steer low-value span root cause analysis

[ArthurKnaus]

Steer Autofix root cause analysis for low-value span issues so Seer treats them as Sentry created optimization guidance instead of exception-style failures. This keeps the normal Autofix step flow while giving root cause analysis enough issue-specific context to use the latest occurrence evidence correctly.

Currently the root cause analysis tries to handle this type of issue as if it is an exception in the user's codebase, not understanding it is an issue created by Sentry to notify users about optimization potential. Low-value span RCA now explains that the root cause is a high-volume span with low telemetry value.

Part of TET-2194

[linear-code]

TET-2194

[github-actions]

Backend Test Failures

Failures on 45e7503 in this run:

tests/sentry/migrations/test_1072_backfill_scm_integration_config.py::BackfillScmIntegrationConfigTest::test — log

Test passed but took too long to run: Duration 139.8306201590001s > 120.0s

tests/sentry/migrations/test_1080_backfill_deprecated_dashboard_widget_display_types.py::BackfillDeprecatedDashboardWidgetDisplayTypesTest::test_backfills — log

Test passed but took too long to run: Duration 126.25163587499992s > 120.0s

tests/sentry/migrations/test_1096_backfill_mcp_dashboard_widget_filters.py::BackfillMcpDashboardWidgetFiltersTest::test — log

Test passed but took too long to run: Duration 123.689076064s > 120.0s

tests/sentry/migrations/test_1103_backfill_auto_link_repos_by_name.py::BackfillAutoLinkReposByNameTest::test — log

Test passed but took too long to run: Duration 126.28881411099997s > 120.0s