chore(aci milestone 3): remove dual write feature flag

[jasonyuezhang]

Remove references to organizations:workflow-engine-metric-alert-dual-write and set its default value to True.

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Copied from getsentry#101899
Original PR: getsentry#101899

[propel-test-bot]

Remove Dual Write Feature Flag for Workflow Engine Metric Alerts

This pull request removes explicit checks for the organizations:workflow-engine-metric-alert-dual-write feature flag throughout the codebase. Instead, it sets the default value of this feature flag to True, ensuring dual write behavior for metric alert rules is always enabled by default. All code paths and tests that conditionally referenced this flag are updated to remove feature flag gating, and the flag registration is modified to have default=True.

Key Changes

• Removed all code references and test usages of the organizations:workflow-engine-metric-alert-dual-write feature flag.
• Set the feature flag organizations:workflow-engine-metric-alert-dual-write default to True in src/sentry/features/temporary.py.
• Eliminated conditional logic in src/sentry/incidents/serializers/alert_rule.py that checked for the dual write feature; dual writing is now always performed.
• Cleaned up related test decorators and context managers (@with_feature, self.feature(...)) for the removed feature flag in incident/alert rule test files.

Affected Areas

• src/sentry/incidents/serializers/alert_rule.py
• src/sentry/features/temporary.py
• tests/sentry/incidents/endpoints/test_organization_alert_rule_details.py
• tests/sentry/incidents/test_logic.py
• tests/sentry/incidents/endpoints/test_organization_alert_rule_index.py

This summary was automatically generated by @propel-code-bot