isClassLoadable answers false on threads without a context classloader

### 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.

Start at ReflectUtils.isClassLoadable and inspect how its default thread context classloader reaches DynClasses.builder().loader(null). Reproduce the probe with a null context classloader, then verify that an application class and the optional SLF4J or MySQL integrations are detected through a real defining loader.

Written by the indexing model from the issue text.