refactor: extract Quarkus plugin tests to separate file

Move the Quarkus-specific tests from PluginUpgradeStrategyTest to a new
PluginUpgradeQuarkusTest class to stay well within the 2000-line
Checkstyle limit and leave room for future plugin additions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: gnodet