Add shoulda-matchers 8 appraisal

[anmarchenko]

What does this PR do?

Adds a shoulda-matchers 8 appraisal for the minitest/shoulda-context integration.

RubyGems reports shoulda-matchers 8.0.1 as the latest stable, non-yanked 8.x release. This independently adds minitest-5-shoulda-context-2-shoulda-matchers-8 and its generated Ruby 3.3, 3.4, and 4.0 gemfiles/lockfiles.

Ruby support

shoulda-matchers 8 requires Ruby >= 3.3, so Appraisals and TEST_METADATA exclude Ruby 2.7–3.2. No compatibility changes were required.

Validation

• APPRAISAL_GROUP=minitest-5-shoulda-context-2-shoulda-matchers-8 bundle exec rake 'appraisal:install[3.3,3.4,4.0]' — passed
• Runtime-prefixed spec:minitest_shoulda_context appraisal runs on Ruby 3.3, 3.4, and 4.0 — 2 examples, 0 failures each
• bundle exec rake test:minitest_shoulda_context — 6.x and 8.x appraisals passed on Ruby 3.4
• bundle exec standardrb — 501 files, no offenses
• bundle exec rake steep:check — no type errors
• git diff --cached --check — passed

[datadog-prod-us1-5]

More details

The new appraisal is gated to Ruby 3.3 and newer, matching shoulda-matchers 8's runtime requirement; executable matrix and lockfile checks found no user-facing regression. Full appraisal execution was unavailable because the sandbox lacks the appraisal executable and the repository's git-sourced dependency checkout.

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[datadog-prod-us1-5]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 97.31% (+0.01%)

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[anmarchenko]

Superseded by #547, which now contains both shoulda-matchers 7 and 8 appraisal coverage. Please use #547 for review and merge.