Fix race conditions in spec/system/all_casa_admins/

[vishaltps]

Fixes #6694

Summary

• Replace all non-webpage expects with page-level assertions in spec/system/all_casa_admins/ to fix race conditions in CI
• Replace CasaOrg.find_by DB query with regex path match on the page
• Remove ActionMailer::Base.deliveries checks (email delivery content is not visible on the webpage; existing page assertions already confirm success)
• Remove CasaAdmin.find_by invitation check (already confirmed by page assertion "New admin created successfully")
• Remove "admin invitations expire" test (pure model test with zero browser interaction — does not belong in a system spec)
• Rewrite expect { visit "/flipper" }.to raise_error(ActionController::RoutingError) to check the webpage by temporarily enabling exception rendering

Files checked

All 5 files from the issue checklist were reviewed:

File	Result
all_casa_admin_spec.rb	Fixed — 5 non-webpage expects replaced/removed
sessions/new_spec.rb	Fixed — 1 non-webpage expect rewritten
edit_spec.rb	Clean — all expects already check the page
password_change_spec.rb	Clean — all expects already check the page
patch_notes/index_spec.rb	Clean — all expects already check the page

Follow-up

The "admin invitations expire" test was removed from the system spec. It should be re-added as a model/unit test in spec/models/all_casa_admin_spec.rb (which does not currently exist).

Test plan

• bundle exec rspec spec/system/all_casa_admins/ — all 17 examples pass, 0 failures