test: Pull in minitest-mock for use with minitest 6

[flavorjones]

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[Copilot]

Pull request overview

This PR updates the test/development dependency set to run the suite against Minitest 6, explicitly adding minitest-mock alongside the Minitest upgrade, and refreshes a few related toolchain gems.

Changes:

• Upgrade minitest to 6.0.6 and add minitest-mock (5.27.0) for mock support.
• Bump supporting dev/test dependencies (debug, minitest-parallel_fork, sqlite3) and RuboCop plugins (rubocop-minitest, rubocop-rails).
• Apply the same dependency updates across the main Gemfile and the Appraisal-generated Rails appraisal Gemfiles.

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Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
Gemfile	Pins Minitest 6 and adds minitest-mock; bumps related dev/test gems.
Gemfile.lock	Locks the updated dependency graph including minitest-mock.
gemfiles/rails_8_1.gemfile	Mirrors the Minitest/dev/test and RuboCop gem version updates for the Rails 8.1 appraisal.
gemfiles/rails_edge.gemfile	Mirrors the Minitest/dev/test and RuboCop gem version updates for the Rails edge appraisal.

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