feat: implement head and headOption operators

The `headOption` operator returns the first element in `Source` wrapped
in `Some` or `None` in case when source is empty or failed e.g.:

  Source.empty[Int].headOption()       // None
  Source.fromValues(1, 2).headOption() // Some(1)

The `head` operator returns the first element in `Source` or throws
`NoSuchElementException` in case when it is either empty or `receive()`
operation fails without error. In case when `receive()` fails with
exception then this exception is re-thrown e.g.:

  Source.empty[Int].head()       // throws NoSuchElementException("cannot obtain head from the empty source")
  Source.fromValues(1, 2).head() // 1

Note that neither `head` nor `headOption` are idempotent operations.

core/src/main/scala/ox/channels/SourceOps.scala

@@ -5,6 +5,7 @@ import ox.*
 import java.util.concurrent.{CountDownLatch, Semaphore}
 import scala.collection.{IterableOnce, mutable}
 import scala.concurrent.duration.FiniteDuration
+import scala.util.Try
 
 trait SourceOps[+T] { this: Source[T] =>
   // view ops (lazy)
@@ -513,6 +514,57 @@ trait SourceOps[+T] { this: Source[T] =>
       }
     }
     c
+
+  /** Returns the first element from this source wrapped in `Some` or `None` when the source is empty or fails during the receive operation.
+    * Note that `headOption` is not an idempotent operation on source as it receives elements from it.
+    *
+    * @return
+    *   A `Some(first element)` if source is not empty or None` otherwise.
+    * @example
+    *   {{{
+    *   import ox.*
+    *   import ox.channels.Source
+    *
+    *   scoped {
+    *     Source.empty[Int].headOption()  // None
+    *     val s = Source.fromValues(1, 2)
+    *     s.headOption()                  // Some(1)
+    *     s.headOption()                  // Some(2)
+    *   }
+    *   }}}
+    */
+  def headOption(): Option[T] = Try(head()).toOption
+
+  /** Returns the first element from this source or throws `NoSuchElementException` when the source is empty or `receive()` operation fails
+    * without error. In case when the `receive()` operation fails with exception that exception is re-thrown. Note that `headOption` is not
+    * an idempotent operation on source as it receives elements from it.
+    *
+    * @return
+    *   A first element if source is not empty or throws otherwise.
+    * @throws NoSuchElementException
+    *   When source is empty or `receive()` failed without error.
+    * @throws exception
+    *   When `receive()` failed with exception then this exception is re-thrown.
+    * @example
+    *   {{{
+    *   import ox.*
+    *   import ox.channels.Source
+    *
+    *   scoped {
+    *     Source.empty[Int].head()        // throws NoSuchElementException("cannot obtain head from the empty source")
+    *     val s = Source.fromValues(1, 2)
+    *     s.head()                        // 1
+    *     s.head()                        // 2
+    *   }
+    *   }}}
+    */
+  def head(): T =
+    supervised {
+      receive() match
+        case ChannelClosed.Done     => throw new NoSuchElementException("cannot obtain head from an empty source")
+        case ChannelClosed.Error(r) => throw r.getOrElse(new NoSuchElementException("getting head failed"))
+        case t: T @unchecked        => t
+    }
 }
 
 trait SourceCompanionOps:
@@ -740,3 +792,13 @@ trait SourceCompanionOps:
     val c = DirectChannel[T]()
     c.error(t)
     c
+
+  /** Creates a source that fails immediately
+    *
+    * @return
+    *   A source that would fail immediately
+    */
+  def failed[T](): Source[T] =
+    val c = DirectChannel[T]()
+    c.error(None)
+    c

core/src/test/scala/ox/channels/SourceOpsHeadOptionTest.scala

@@ -0,0 +1,30 @@
+package ox.channels
+
+import org.scalatest.OptionValues
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+import ox.*
+
+class SourceOpsHeadOptionTest extends AnyFlatSpec with Matchers with OptionValues {
+  behavior of "Source.headOption"
+
+  it should "return None for the empty source" in supervised {
+    Source.empty[Int].headOption() shouldBe None
+  }
+
+  it should "return None for the failed source" in supervised {
+    Source
+      .failed(new RuntimeException("source is broken"))
+      .headOption() shouldBe None
+  }
+
+  it should "return Some element for the non-empty source" in supervised {
+    Source.fromValues(1, 2).headOption().value shouldBe 1
+  }
+
+  it should "be not idempotent operation" in supervised {
+    val s = Source.fromValues(1, 2)
+    s.headOption().value shouldBe 1
+    s.headOption().value shouldBe 2
+  }
+}

core/src/test/scala/ox/channels/SourceOpsHeadTest.scala

@@ -0,0 +1,39 @@
+package ox.channels
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+import ox.*
+
+class SourceOpsHeadTest extends AnyFlatSpec with Matchers {
+  behavior of "Source.head"
+
+  it should "throw NoSuchElementException for the empty source" in supervised {
+    the[NoSuchElementException] thrownBy {
+      Source.empty[Int].head()
+    } should have message "cannot obtain head from an empty source"
+  }
+
+  it should "re-throw exception that was thrown during element retrieval" in supervised {
+    the[RuntimeException] thrownBy {
+      Source
+        .failed(new RuntimeException("source is broken"))
+        .head()
+    } should have message "source is broken"
+  }
+
+  it should "throw NoSuchElementException for source failed without exception" in supervised {
+    the[NoSuchElementException] thrownBy {
+      Source.failed[Int]().head()
+    } should have message "getting head failed"
+  }
+
+  it should "return first value from non empty source" in supervised {
+    Source.fromValues(1, 2).head() shouldBe 1
+  }
+
+  it should "be not idempotent operation" in supervised {
+    val s = Source.fromValues(1, 2)
+    s.head() shouldBe 1
+    s.head() shouldBe 2
+  }
+}