Add automated tests for generated RPC code to restore JaCoCo coverage ≥85%

src/test/java/com/github/copilot/sdk/TestUtil.java

@@ -10,13 +10,27 @@
 import java.nio.file.Paths;
 
 /**
- * Shared test utilities for locating the Copilot CLI binary.
+ * Shared test utilities for locating the Copilot CLI binary and other
+ * cross-platform test helpers.
  */
-final class TestUtil {
+public final class TestUtil {
 
     private TestUtil() {
     }
 
+    /**
+     * Returns a platform-independent path string for a file inside the system
+     * temporary directory. Uses {@code java.io.tmpdir} so tests run correctly on
+     * both POSIX and Windows.
+     *
+     * @param filename
+     *            the file name (no directory separator required)
+     * @return absolute path string in the system temp directory
+     */
+    public static String tempPath(String filename) {
+        return Path.of(System.getProperty("java.io.tmpdir"), filename).toString();
+    }
+
     /**
      * Locates a launchable Copilot CLI executable.
      * <p>