refactor(cdn): CDN URL 변환 함수를 공통화한다

src/main/kotlin/kr/co/vividnext/sodalive/v2/common/domain/CdnUrlExtensions.kt

@@ -0,0 +1,7 @@
+package kr.co.vividnext.sodalive.v2.common.domain
+
+fun String?.toCdnUrl(cloudFrontHost: String): String? {
+    if (isNullOrBlank()) return null
+    if (startsWith("https://") || startsWith("http://")) return this
+    return "$cloudFrontHost/$this"
+}

src/test/kotlin/kr/co/vividnext/sodalive/v2/common/domain/CdnUrlExtensionsTest.kt

@@ -0,0 +1,39 @@
+package kr.co.vividnext.sodalive.v2.common.domain
+
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.DisplayName
+import org.junit.jupiter.api.Test
+
+class CdnUrlExtensionsTest {
+    private val cloudFrontHost = "https://cdn.test"
+
+    @Test
+    @DisplayName("CDN URL 변환은 null과 blank를 null로 반환한다")
+    fun shouldReturnNullForNullOrBlankPath() {
+        assertEquals(null, null.toCdnUrl(cloudFrontHost))
+        assertEquals(null, "".toCdnUrl(cloudFrontHost))
+        assertEquals(null, " ".toCdnUrl(cloudFrontHost))
+    }
+
+    @Test
+    @DisplayName("CDN URL 변환은 절대 URL을 그대로 반환한다")
+    fun shouldKeepAbsoluteUrl() {
+        assertEquals(
+            "https://image.test/profile.png",
+            "https://image.test/profile.png".toCdnUrl(cloudFrontHost)
+        )
+        assertEquals(
+            "http://image.test/profile.png",
+            "http://image.test/profile.png".toCdnUrl(cloudFrontHost)
+        )
+    }
+
+    @Test
+    @DisplayName("CDN URL 변환은 상대 path 앞에 CloudFront host를 붙인다")
+    fun shouldPrependCloudFrontHostToRelativePath() {
+        assertEquals(
+            "https://cdn.test/profile/default-profile.png",
+            "profile/default-profile.png".toCdnUrl(cloudFrontHost)
+        )
+    }
+}