test: improve infection coverage

[Max Stegmeyer (mstegmeyer)]

Summary

• upgrade the remaining workflows from actions/checkout@v2 to @v4 to remove the Node 20 deprecation warning
• add targeted mutation-killing tests around DynamoDB shop persistence, dual-signature fallback behavior, context resolver fallbacks, authenticated client token validation, payment responses, and storefront shop resolution

Why

The Infection job on main was reporting multiple escaped mutants plus the checkout deprecation warning. These changes remove the covered survivors and raise the mutation threshold to match the improved score.

Validation

• vendor/bin/phpunit
• devenv shell -- vendor/bin/infection --min-msi=99 --min-covered-msi=100 --threads=4