ref(notifications): Create NotificationRule to be used in place of punned Rule

[kcons]

Instead of reusing Rule, introduce a new type that can be created from a Rule or not, and provide a easy way to identify where Rules are still being used so we can remove them.

[github-actions]

Backend Test Failures

Failures on 00da073 in this run:

tests/sentry/digests/test_utilities.py::GetPersonalizedDigestsTestCase::test_team_without_members_with_legacy_rule_id — log

[gw1] linux -- Python 3.13.15 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/digests/test_utilities.py:397: in test_team_without_members_with_legacy_rule_id
    assert_rule_ids(digest, [rule.data["actions"][0]["legacy_rule_id"]])
tests/sentry/digests/test_utilities.py:119: in assert_rule_ids
    assert rule.id in expected_rule_ids
           ^^^^^^^
E   AttributeError: 'NotificationRule' object has no attribute 'id'

tests/sentry/integrations/slack/test_message_builder.py::BuildGroupAttachmentTest::test_build_group_block_with_workflow_only — log

[gw1] linux -- Python 3.13.15 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/integrations/slack/test_message_builder.py:465: in test_build_group_block_with_workflow_only
    blocks = SlackIssuesMessageBuilder(self.group, rules=[rule]).build()["blocks"]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
src/sentry/integrations/slack/message_builder/issues.py:610: in build
    key, value = get_rule_or_workflow_id(self.rules[0])
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
src/sentry/notifications/utils/rules.py:93: in get_rule_or_workflow_id
    if rule.legacy_rule_id is not None:
       ^^^^^^^^^^^^^^^^^^^
E   AttributeError: 'Rule' object has no attribute 'legacy_rule_id'

tests/sentry/digests/test_utilities.py::GetPersonalizedDigestsTestCase::test_direct_email_with_legacy_rule_id — log

[gw0] linux -- Python 3.13.15 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/digests/test_utilities.py:328: in test_direct_email_with_legacy_rule_id
    assert_rule_ids(
tests/sentry/digests/test_utilities.py:119: in assert_rule_ids
    assert rule.id in expected_rule_ids
           ^^^^^^^
E   AttributeError: 'NotificationRule' object has no attribute 'id'

tests/sentry/digests/test_utilities.py::GetPersonalizedDigestsTestCase::test_simple_with_legacy_rule_id — log

[gw0] linux -- Python 3.13.15 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/digests/test_utilities.py:287: in test_simple_with_legacy_rule_id
    assert_rule_ids(
tests/sentry/digests/test_utilities.py:119: in assert_rule_ids
    assert rule.id in expected_rule_ids
           ^^^^^^^
E   AttributeError: 'NotificationRule' object has no attribute 'id'