[I] isClassLoadable answers false on threads without a context classloader [kyuubi]

["LuciferYang (via GitHub)" <gi...@apache.org>]

LuciferYang opened a new issue, #7733:
URL: https://github.com/apache/kyuubi/issues/7733

   ### Describe the bug
   
   `ReflectUtils.isClassLoadable` defaults its loader to the thread context classloader:
   
   ```scala
   def isClassLoadable(
       className: String,
       cl: ClassLoader = Thread.currentThread().getContextClassLoader): Boolean = ...
   ```
   
   On a thread whose context classloader is null, `DynClasses.builder().loader(null)` resolves the name with `Class.forName(name, true, null)`, which delegates to the bootstrap loader. The bootstrap loader sees only JDK classes, so the probe answers `false` for every application class, including Kyuubi's own, even when that class is loadable through the loader that defined `ReflectUtils`.
   
   A thread can carry a null context classloader in embedded scenarios: a host that starts the JVM through the JNI invocation API without setting one, or a container framework that clears it. `Logging` and `JDBCMetadataStore` probe optional integrations through this helper, so on such a thread the SLF4J bridge or the MySQL driver would be reported as absent when it is present.
   
   Spark's own `Utils.classForName` / `getContextOrSparkClassLoader` guard against this by falling back to a real loader instead of trusting a null context loader.
   
   ### Affects Version(s)
   
   master (1.13.0-SNAPSHOT), and earlier releases carrying `kyuubi-util-scala`.
   
   ### Are you willing to submit PR?
   
   Yes.
   


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