pkl-doc: Fix/improve Executor handling in DocGenerator (#1590)

run() now creates and closes a default Executor per call. This is fine
because there is no good reason to call this method multiple times.

run(Executor) now lets callers provide their own Executor, which is
customary for a well-behaved library.

Also: Fix IntelliJ warning by calling toSet()

Closes #1583

pkl-doc/src/test/kotlin/org/pkl/doc/DocGeneratorTest.kt

@@ -17,22 +17,15 @@ package org.pkl.doc
 
 import org.assertj.core.api.Assertions.assertThat
 import org.junit.jupiter.api.Test
-import org.junit.jupiter.api.condition.EnabledIf
+import org.junit.jupiter.api.condition.EnabledForJreRange
+import org.junit.jupiter.api.condition.JRE
 
 class DocGeneratorTest {
-  companion object {
-    @JvmStatic
-    fun isJdk21OrLater(): Boolean {
-      return Runtime.version().feature() >= 21
-    }
-  }
-
   @Test
-  @EnabledIf("isJdk21OrLater")
-  fun `uses virtual thread executor on JDK 21`() {
+  @EnabledForJreRange(min = JRE.JAVA_21)
+  fun `uses virtual thread executor on JDK 21+`() {
     // On older JDKs, we get a ThreadPoolExecutor.
-    // not sure if there's a better assertion to make here.
-    assertThat(DocGenerator.executor.javaClass.canonicalName)
+    assertThat(DocGenerator.createDefaultExecutor().javaClass.canonicalName)
       .isEqualTo("java.util.concurrent.ThreadPerTaskExecutor")
   }
 }