Close and unregister plugin classloader when load fails

[newPlayerAL]

When a plugin fails to load during Class.forName / newInstance (e.g. a bad main class, an exception in the plugin constructor), the PluginClassLoader had already been registered in pluginLoaders (and the dependency list in pluginDependencies) before instantiation was attempted. On failure the exception propagated out without closing the classloader or removing the map entries, leaking the open jar file handle. On Windows this surfaces as "file is in use" when reloading the plugin.

Changes

• PluginManager.loadPlugin(File) and instantiateAndLoad: wrap the Class.forName / newInstance in try/catch; on failure remove the pluginLoaders and pluginDependencies entries, close() the classloader, then rethrow.
• Added PluginClassloaderLeakTest: loads a jar whose plugin.yml is valid but whose main points to a non-existent class, asserts the load throws and that no classloader / dependency entry lingers afterwards.
• Registered the new test in maven-test.yml.

No behavior change on the successful-load path.

[codacy-production]

Not up to standards ⛔

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