test: cover DeliveryMethods::Sms and CustomOrgLinkPolicy::Scope (#6833)

[augustocbx]

What github issue is this PR for, if any?

Part of #6833

What changed, and why?

Issue #6833 lists app/notifications/delivery_methods/sms.rb (43.8% line coverage) and app/policies/custom_org_link_policy.rb (50%) among the remaining low-coverage Ruby files (controllers were already handled in #6841 and #6855). This PR covers both:

• spec/notifications/delivery_methods/sms_spec.rb — replaced the pending stub with real specs: an SMS with the flagged-case-contact message and the shortened case contact edit URL is sent via Twilio when the sender is a casa admin or supervisor, and nothing is sent when the sender is a volunteer. Uses the existing WebMockHelper short.io/Twilio stubs.
• spec/policies/custom_org_link_policy_spec.rb — added Scope#resolve specs (the previously uncovered half of the policy): casa admins resolve only their own organization's links, supervisors and volunteers resolve none.
• app/notifications/delivery_methods/sms.rb — memoized #sender, which called User.find on every invocation (3 queries per delivery). Prosopite, recently enabled for spec/notifications in Scan /spec/notifications for N+1s #6994, flags this as an N+1 and fails the new specs without the memoization.

How is this tested? (please write rspec and jest tests!) 💖💪

• bundle exec rspec spec/notifications/delivery_methods/sms_spec.rb spec/policies/custom_org_link_policy_spec.rb — 10 examples, 0 failures
• bundle exec rspec spec/notifications spec/policies spec/services/twilio_service_spec.rb spec/services/short_url_service_spec.rb spec/services/followup_service_spec.rb — 404 examples, 0 failures, 9 pending
• bundle exec standardrb on the three touched files — no offenses

The new SMS specs fail without the memoization change (Prosopite raises NPlusOneQueriesError) and pass with it.

Screenshots please :)

No UI changes — test coverage plus a query memoization.