Fix registration_policy changes not captured in event changelog

[nbudin]

Summary

• Fixes a bug where updating an event's registration policy via the UpdateEvent mutation would record the same value for both previous_value and new_value in the FormResponseChange changelog entry
• The root cause: apply_registration_policy called event.reload after EventChangeRegistrationPolicyService, which overwrote the in-memory event with the saved new policy — so event.registration_policy was already the new policy when we tried to capture the old one
• Fix: capture old_registration_policy = event.registration_policy before calling the service and reload

Test plan

• Added test/graphql/mutations/update_event_test.rb with tests that verify:
  • A FormResponseChange is created for registration_policy changes
  • previous_value is the old policy, not the new one
  • new_value is the new policy
  • previous_value and new_value are different
  • Registration policy changes flow through to NotifyFormResponseChangesService
• Added test/graphql/mutations/update_event_proposal_test.rb with tests that verify:
  • Proposal registration_policy changes are also captured correctly
  • Unchanged registration_policy does not create a spurious FormResponseChange

Fixes #10998

🤖 Generated with Claude Code

[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
app/graphql/mutations/base_mutation.rb	🟠 67.24%	🟢 75.86%	🟢 8.62%
app/graphql/mutations/update_event.rb	🔴 35.71%	🟢 100%	🟢 64.29%
app/graphql/mutations/update_event_proposal.rb	🔴 42.86%	🟢 100%	🟢 57.14%
app/models/concerns/form_response.rb	🟢 86.36%	🟢 93.18%	🟢 6.82%
app/models/form_item.rb	🟢 85.45%	🟢 90.91%	🟢 5.46%
app/policies/event_policy.rb	🟢 98.53%	🟢 100%	🟢 1.47%
app/policies/event_proposal_policy.rb	🟢 93.33%	🟢 100%	🟢 6.67%
app/services/compact_form_response_changes_service.rb	🔴 0%	🔴 46.15%	🟢 46.15%
app/services/notify_form_response_changes_service.rb	🔴 0%	🟢 100%	🟢 100%
test/graphql/mutations/update_event_proposal_test.rb	🔴 0%	🟢 100%	🟢 100%
test/graphql/mutations/update_event_test.rb	🔴 0%	🟢 100%	🟢 100%
Overall Coverage	🟢 52.75%	🟢 53.02%	🟢 0.27%

Minimum allowed coverage is 0%, this run produced 53.02%