Add email support test case to mailer shared examples

[aduth]

🛠 Summary of changes

Enhances existing mailer shared example specs to include checks for content which may not be broadly supported in email clients. Currently, this includes a check for use of SVG images, which would be encouraged in application views, but which does not have broad support in email clients.

Previously:

• LG-8887: In-Person Proofing: Add Login.gov logo above the Barcode #7956 (comment)
• Updating mailer image from svg to png for Google #6892
• LG-6343: Implement email template for "Ready to verify" #6585 (comment)

📜 Testing Plan

Trigger a failure by adding a violating image to an email template in your local copy of the branch.

Example:

diff --git a/app/views/user_mailer/add_email.html.erb b/app/views/user_mailer/add_email.html.erb
index 6b31c0f2f..527bcd73c 100644
--- a/app/views/user_mailer/add_email.html.erb
+++ b/app/views/user_mailer/add_email.html.erb
@@ -3,6 +3,7 @@
   <%= t('user_mailer.add_email.footer', confirmation_period: @confirmation_period) %>
 </p>
 
+<%= image_tag(asset_url('logo.svg')) %>
 <table class="button expanded large radius">
   <tbody>
     <tr>

$ rspec spec/mailers/user_mailer_spec.rb 

Failures:

  1) UserMailer#add_email behaves like a system email does not include markup or layout with lack of broad email support
     Failure/Error: expect(body).not_to have_css('img[src$=".svg"]')
       expected not to find visible css "img[src$=\".svg\"]", found 1 match: ""
     Shared Example Group: "a system email" called from ./spec/mailers/user_mailer_spec.rb:32
     # ./spec/support/shared_examples_for_mailer.rb:10:in `block (2 levels) in <main>'

[allthesignals]

Awesome!

[allthesignals]

Any other common mailer/web support discrepancies that might have come up in the past?

[aduth]

Any other common mailer/web support discrepancies that might have come up in the past?

I'm sure there are, but none which come to mind at the moment. But I left the test description intentionally broad so that we can add more assertions as we encounter them.