Add regression coverage for packager validation paths

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Summary

• Added focused JUnit coverage for commit descriptor validation in PluginTest:
  • non-GitHub repository clone URLs are rejected with actionable help text
  • misspelled/unknown descriptor keys fail fast with file-line context
  • disabled= and unavailable= descriptors preserve their different unavailable-manifest semantics
• Added UploadConfigurationTest coverage for recursive WebDAV MKCOL behavior when a child directory returns 409 before its parent exists.

Risky Behavior Now Covered

• Plugin descriptor parsing gates every plugin build; URL validation and typo rejection prevent silently accepting invalid or misspelled metadata.
• Disabled vs unavailable descriptors control whether broken plugins are only skipped during packaging or also surfaced in unavailable metadata.
• Upload directory creation must create missing parent directories before retrying child paths, or package uploads can fail during publication.

Test Files Added/Updated

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

Why This Reduces Regression Risk

• These tests cover high-blast-radius validation and upload code paths rather than low-signal plugin manifest hash updates.
• The assertions exercise real constructor and HTTP client behavior with existing JUnit/MockWebServer conventions.
• Mutation checks confirmed the new tests fail if unknown descriptor keys are accepted or 409 parent-directory recovery is disabled.

Validation

• ./gradlew prep succeeded.
• Targeted tests passed:
  • ./gradlew :package:test --tests net.runelite.pluginhub.packager.PluginTest.testRepositoryMustBeGithubCloneUrl --tests net.runelite.pluginhub.packager.PluginTest.testUnexpectedCommitDescriptorKeyFailsFast --tests net.runelite.pluginhub.packager.PluginTest.testDisabledCommitDescriptorIsExcludedFromUnavailable --tests net.runelite.pluginhub.packager.PluginTest.testUnavailableCommitDescriptorIsIncludedInUnavailable :upload:test --tests net.runelite.pluginhub.uploader.UploadConfigurationTest
• Mutation checks failed as expected, then passed after restoring production files.
• Note: running the full PluginTest class still hits pre-existing generated example-plugin compile failures unrelated to these new constructor-level tests.