chore(ci): enable automated doctest execution across gem CI matrix

[torreypayne]

📚 PR Stack Navigation

1. PR #1: Zizmor Security Audit Fixes (Base)
2. PR #2: Doctest Prefactoring (Sample Loader & 11-Gem Mocks)
3. 👉 PR #3: Doctest CI Matrix Switch (Top of Stack - You are here)

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Enables automated YARD doctest execution in the Ubuntu Ruby 4.0 CI matrix.

Stack:

1. chore(ci): address zizmor findings #36206 (Zizmor security fixes)
2. refactor(doctest): prefactor sample loader and helper mocks for CI compatibility #36295 (Prefactoring: sample loader & doctest helper mocks across 11 gems)
3. This PR: Enables CI matrix switch in ci.yml & ci.rb (2-line diff)

Fixes: b/434017709

[aandreassa]

Files changed:
  .github/.OwlBot.lock.yaml
  .github/workflows/ci.yml
  .toys/ci.rb
[2026-08-18 22:02:54  INFO]  exec: ["bundle", "install", "--retry=3"]
No gem directories changed.

doctest didn't run in CI. It would be good to trigger as a demo with the libraries you are fixing.