Omit superfluous newline when writing formatted content to stdout (#1350)

Fixes an issue where an extra newline is added when writing formatted
code to stdout.

Closes https://github.com/apple/pkl/issues/1346

pkl-cli/src/test/kotlin/org/pkl/cli/CliFormatterTest.kt

@@ -0,0 +1,47 @@
+/*
+ * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pkl.cli
+
+import java.nio.file.Path
+import kotlin.io.path.writeText
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.io.TempDir
+import org.pkl.commons.cli.CliException
+import org.pkl.core.util.StringBuilderWriter
+import org.pkl.formatter.GrammarVersion
+
+class CliFormatterTest {
+  @Test
+  fun `no double newline when writing to stdout`(@TempDir tempDir: Path) {
+    val file = tempDir.resolve("foo.pkl").also { it.writeText("foo = 1") }
+    val sb = StringBuilder()
+    val writer = StringBuilderWriter(sb)
+    val cmd =
+      CliFormatterCommand(
+        listOf(file),
+        GrammarVersion.latest(),
+        overwrite = false,
+        diffNameOnly = false,
+        silent = false,
+        consoleWriter = writer,
+      )
+    try {
+      cmd.run()
+    } catch (_: CliException) {}
+    assertThat(sb.toString()).isEqualTo("foo = 1\n")
+  }
+}