Add regression coverage for missing plugin commit descriptors

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Summary

• Add a focused regression test for plugin descriptor files that omit the required commit key.
• Return a controlled PluginBuildException for missing commits instead of allowing a NullPointerException.

Risky Behavior Now Covered

• Plugin marker descriptors missing commit= now fail through the normal validation path with actionable error context.

Test Files Added/Updated

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

Why This Reduces Regression Risk

• Recent merged changes are mostly plugin marker additions/updates, so descriptor validation is the core business-risk surface.
• The test prevents malformed descriptors from crashing package validation and preserves helpful CI feedback for contributors.

Validation

• ./gradlew --console=plain prep
• ./gradlew --console=plain :package:test --tests net.runelite.pluginhub.packager.PluginTest.testCommitMustBePresent first failed red with NullPointerException, then passed after the fix.
• ./gradlew --console=plain :package:compileJava :package:compileTestJava :package:test --tests net.runelite.pluginhub.packager.PluginTest.testCommitMustBePresent passed.

Notes

• Full ./gradlew --console=plain :package:test was attempted but existing example-plugin compile tests fail in this environment under Java 21; CI uses Java 11 per .github/workflows/build.yml.