refactor(tests): replace mailer mocks with email delivery assertions

[mroderick]

Summary

Replaces mock-based mailer tests with assertions on ActionMailer::Base.deliveries. This improves test isolation and eliminates flaky tests in parallel test execution.

Problem

The existing tests used expect(Mailer).to receive(:method) and expect_any_instance_of(Mailer) which:

• Cause race conditions in parallel test runs (same mock catches calls from other processes)
• Test implementation details rather than behavior
• Are globally scoped, causing interference between tests

Solution

Replace mock expectations with assertions on actual email delivery:

• Verify email count: expect { action }.to change { ActionMailer::Base.deliveries.count }
• Verify email recipients: expect(email.to).to include(member.email)
• Verify email content: expect(email.body.encoded).to match(/pattern/)

Files Changed

File	Changes
spec/support/shared_examples/behaves_like_sending_workshop_emails.rb	Replace mailer mocks with delivery count assertions
spec/models/invitation_manager_spec.rb	Convert all mailer mocks (waiting list, reminders, meetings, async)
spec/models/feedback_request_spec.rb	Simplify to single email delivery test
spec/models/eligibility_inquiry_spec.rb	Replace spy with delivery assertion
spec/models/attendance_warning_spec.rb	Replace spy with delivery assertion
spec/features/subscribing_to_emails_spec.rb	Replace all expect_any_instance_of mocks
spec/mailers/member_mailer_spec.rb	Replace expect_any_instance_of with content verification

Test Results

All 59 affected tests pass:

• 37 invitation_manager tests
• 10 invitation_manager_logging tests
• 7 feedback_request tests
• 2 eligibility_inquiry tests
• 3 attendance_warning tests
• 8 feature tests (subscriptions)
• 15 mailer tests

Benefits

1. Better parallel test isolation - No shared global state from mocks
2. Tests behavior, not implementation - Verifies emails are actually sent
3. More resilient - Won't break if internal method names change
4. Clearer intent - Tests describe what should happen, not how

[mroderick]

@olleolleolle is this the most Rails idiomatic way of testing email sending?