Add regression coverage for plugin descriptor parsing

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Summary

• Added a PluginTest regression test that loads every descriptor in plugins/ through the production Plugin constructor.
• Treats disabled/unavailable descriptors as the packager does, while failing on malformed active descriptors.

Risky Behavior Now Covered

• Recently added or updated plugin descriptor files must still satisfy packager parser rules for internal name, repository URL, commit SHA, optional metadata, and unexpected keys.
• Disabled and unavailable plugin descriptors remain allowed to short-circuit without requiring active build metadata.

Test Files Added/Updated

• package/package/src/test/java/net/runelite/pluginhub/packager/PluginTest.java

Why This Reduces Regression Risk

• Recent merged changes were primarily plugin descriptor additions and updates, which directly determine what the packager builds or suppresses.
• The test exercises the same constructor path used by the build flow, so descriptor typos or schema drift fail deterministically before they can affect packaging output.

Validation

• Passed: ./gradlew :package:test --tests net.runelite.pluginhub.packager.PluginTest.testAllPluginDescriptorsParse --console=plain
• Attempted: ./gradlew prep :package:test --console=plain; failed in pre-existing example-plugin compile tests under this VM's Java 21 runtime. The repository workflow config runs Java 11, and the new descriptor parser test passed independently.