Use correct parent classloader for worksheets.

Instead of passing in `null` when we extend ClassLoader in our
MdocClassLoader this passes in the actual parent since on Java +9 when
using `null` that's the bootstrap classloader which doesn't contain
modules like java.sql.

Big thanks to @smarter pointing this out in scala/scala3#11658.

Fixes scalameta#2187

Author: ckipp01

metals/src/main/scala/scala/meta/internal/worksheets/WorksheetClassLoader.scala

@@ -8,7 +8,8 @@ package scala.meta.internal.worksheets
  * allowing the Metals server to call mdoc instances from different Scala
  * versions.
  */
-class MdocClassLoader(parent: ClassLoader) extends ClassLoader(null) {
+class MdocClassLoader(parent: ClassLoader)
+    extends ClassLoader(ClassLoader.getSystemClassLoader.getParent) {
   override def findClass(name: String): Class[_] = {
     val isShared =
       name.startsWith("mdoc.interfaces") || name.startsWith("coursierapi")

tests/unit/src/test/scala/tests/worksheets/WorksheetLspSuite.scala

@@ -91,4 +91,27 @@ class WorksheetLspSuite extends tests.BaseWorksheetLspSuite(V.scala212) {
       )
     } yield ()
   }
+  // Ensure that on Java +9 that all modules are correctly loaded with the Mdoc
+  // classloader including things like the java.sql module.
+  // https://github.com/scalameta/metals/issues/2187
+  test("classloader") {
+    val path = "hi.worksheet.sc"
+    for {
+      _ <- server.initialize(
+        s"""
+           |/metals.json
+           |{
+           |  "a": {}
+           |}
+           |/${path}
+           |new java.sql.Date(100L)
+           |""".stripMargin
+      )
+      _ <- server.didOpen(path)
+      _ = assertNoDiff(
+        client.workspaceDecorations,
+        "new java.sql.Date(100L) // : java.sql.Date = 1970-01-01"
+      )
+    } yield ()
+  }
 }