KTOR-9168 Add regression test for case-insensitive Set-Cookie merging in CIO

ktor-client/ktor-client-cio/jvm/test/io/ktor/client/engine/cio/CookieHeaderCaseTest.kt

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2014-2026 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package io.ktor.client.engine.cio
+
+import io.ktor.client.*
+import io.ktor.client.request.*
+import io.ktor.http.*
+import kotlinx.coroutines.test.runTest
+import java.net.ServerSocket
+import java.net.SocketException
+import kotlin.concurrent.thread
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class CookieHeaderCaseTest {
+
+    @Test
+    fun `CIO preserves Set-Cookie headers regardless of case`() = runTest {
+        HttpClient(CIO).use { client ->
+            ServerSocket(0).use { server ->
+                val thread = thread {
+                    try {
+                        server.accept().use { socket ->
+                            socket.getOutputStream().let { out ->
+                                out.write(
+                                    (
+                                        "HTTP/1.1 200 OK\r\n" +
+                                            "Content-Length: 0\r\n" +
+                                            "Set-Cookie: first=1\r\n" +
+                                            "set-cookie: second=2\r\n" +
+                                            "\r\n"
+                                        ).toByteArray()
+                                )
+                                out.flush()
+                            }
+                        }
+                    } catch (_: SocketException) {
+                    }
+                }
+
+                val response = client.get("http://localhost:${server.localPort}/")
+                assertEquals(
+                    listOf("first=1", "second=2"),
+                    response.headers.getAll(HttpHeaders.SetCookie)
+                )
+                thread.join()
+            }
+        }
+    }
+}