PluginManager has fragile dependency on singleton and system properties

There's an issue with how the PluginManager (the only class using PluginClassLoader) gets initialized. It has a static PluginManager variable that uses system properties to locate classes to load, which is fragile as code that sets up this system property must run before the PluginManager class gets loaded. One example of this is `HadoopSegmentCreationMapper`.

In general having a singleton is problematic in an environment like Hadoop/Spark/Flink, where multiple task threads are running in the same JVM.

Start by reading PluginManager and PluginClassLoader, then inspect how HadoopSegmentCreationMapper sets the relevant system property before PluginManager loads. Trace initialization in Hadoop, Spark, and Flink-style multi-task JVM usage. Done means removing the fragile singleton and initialization-order dependency while preserving plugin loading for these environments.

Written by the indexing model from the issue text.