Use capture_emails helper to improve email assertions in specs

[mjankowski]

Related changes here:

• Add ActionMailer::TestHelper to spec config, to make this helper method available
• Remove before/after where we were explicitly resetting the deliveries collection
• Wrap the code that we expect to generate emails with the capture_emails helper
• Update the assertions to be on this set of captured emails
• In a few cases where we were just checking for emails at all, expand coverage a bit more to specify the to recipient and the subject

Main benefit here is increased confidence that the emails we are asserting against came from where we expected them. I think there's a small but nice readability improvement on the assertions as well. Possible future improvements here:

• The auth/sessions block where we exceed the 2FA rate limit is a little awkward/repetitive, might be good to extract methods here at some point
• We are using the email_spec gem helpers in our mailer specs, but not in these controller specs. I did not do this here because I recall some issue with them not being fully chainable -- but this might be good future improvement as well.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (86500e3) 85.01% compared to head (a769d54) 85.01%.
Report is 96 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #29245      +/-   ##
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- Coverage   85.01%   85.01%   -0.01%     
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  Files        1059     1060       +1     
  Lines       28277    28314      +37     
  Branches     4538     4542       +4     
==========================================
+ Hits        24040    24071      +31     
- Misses       3074     3077       +3     
- Partials     1163     1166       +3     

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[ClearlyClaire]

Will actually review next week, but that sounds like a great improvement!

[mjankowski]

Pushed commit to restore the email count checks.