fixed a flaky test (#566)

Sometimes in `OAuthClientSuite.scala`, after invoking server.shutdown(), the HTTP server does not fully stop, leading to the port not being released. This PR adds an additional safeguard to ensure the test server releases the port properly.

client/src/test/scala/io/delta/sharing/client/auth/OAuthClientSuite.scala

@@ -35,15 +35,27 @@ class OAuthClientSuite extends SparkFunSuite {
       .create()
 
     server.start()
-
-    Thread.sleep(2000)
   }
 
   def stopServer(): Unit = {
     if (server != null) {
-      server.shutdown(2, TimeUnit.SECONDS)
+      server.shutdown(5, TimeUnit.SECONDS)
       server = null
     }
+    waitForPortRelease(1080, 10000)
+  }
+
+  def waitForPortRelease(port: Int, timeoutMillis: Int): Unit = {
+    val startTime = System.currentTimeMillis()
+    while (System.currentTimeMillis() - startTime < timeoutMillis) {
+      try {
+        new java.net.Socket("localhost", port).close()
+      } catch {
+        case _: java.net.ConnectException => return // Port is released
+      }
+      Thread.sleep(100) // Wait for 100 milliseconds before checking again
+    }
+    throw new RuntimeException(s"Port $port is not released after $timeoutMillis milliseconds")
   }
 
   test("OAuthClient should parse token response correctly") {