Skip empty upgrade ask prompt

[MikeMcQuaid]

• Avoid prompting for brew upgrade --ask when the dry-run plan has no version changes.
• Keep the ask-flow regression explicit so the prompt still appears when there are upgrades.

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[Copilot]

Pull request overview

This PR updates brew upgrade --ask so it only prompts for confirmation when the dry-run upgrade summary contains actual version changes.

Changes:

• Gates the upgrade confirmation prompt on final_upgrade_summary.version_changes.
• Updates ask-flow tests to cover both upgrade-present and no-upgrade cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Library/Homebrew/cmd/upgrade.rb	Skips Install.ask when the dry-run plan has no upgrade version changes.
Library/Homebrew/test/cmd/upgrade_spec.rb	Adjusts and adds regression tests for --ask prompt behavior.

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