Improve API of interface WellknownRetriever to be able to distinguish between 404 and other errors.

libraries/wellknown/api/src/main/kotlin/io/element/android/libraries/wellknown/api/SessionWellknownRetriever.kt

@@ -8,6 +8,6 @@
 package io.element.android.libraries.wellknown.api
 
 interface SessionWellknownRetriever {
-    suspend fun getWellKnown(): WellKnown?
-    suspend fun getElementWellKnown(): ElementWellKnown?
+    suspend fun getWellKnown(): WellknownRetrieverResult<WellKnown>
+    suspend fun getElementWellKnown(): WellknownRetrieverResult<ElementWellKnown>
 }

libraries/wellknown/api/src/main/kotlin/io/element/android/libraries/wellknown/api/WellknownRetriever.kt

@@ -8,6 +8,6 @@
 package io.element.android.libraries.wellknown.api
 
 interface WellknownRetriever {
-    suspend fun getWellKnown(baseUrl: String): WellKnown?
-    suspend fun getElementWellKnown(baseUrl: String): ElementWellKnown?
+    suspend fun getWellKnown(baseUrl: String): WellknownRetrieverResult<WellKnown>
+    suspend fun getElementWellKnown(baseUrl: String): WellknownRetrieverResult<ElementWellKnown>
 }

libraries/wellknown/api/src/main/kotlin/io/element/android/libraries/wellknown/api/WellknownRetrieverResult.kt

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2025 New Vector Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.libraries.wellknown.api
+
+sealed interface WellknownRetrieverResult<out T> {
+    /**
+     * Well-known data has been successfully retrieved.
+     */
+    data class Success<out T>(val data: T) : WellknownRetrieverResult<T>
+
+    /**
+     * Well-known data is not found (file does not exist server side, we got a 404).
+     */
+    data object NotFound : WellknownRetrieverResult<Nothing>
+
+    /**
+     * Any other error.
+     */
+    data class Error(val exception: Exception) : WellknownRetrieverResult<Nothing>
+
+    fun dataOrNull(): T? = when (this) {
+        is Success<T> -> data
+        is Error -> null
+        NotFound -> null
+    }
+}