test(nostr): unit test Blossom.buildServerListTags

app/src/main/kotlin/com/wisp/app/nostr/Blossom.kt

@@ -26,14 +26,20 @@ object Blossom {
             if (tag.size >= 2 && tag[0] == "server") {
                 val url = tag[1].toHttpUrlOrNull()
                 url?.toString()
-            }
-            else {
+            } else {
                 null
             }
         }.distinct()
     }
 
+    /**
+     * Builds BUD-03 server tags from a non-empty list of unique already-valid server URLs.
+     * URL validation is expected to happen when URLs enter repository/viewmodel state.
+     *
+     * @throws IllegalArgumentException if [urls] is empty.
+     */
     fun buildServerListTags(urls: List<String>): List<List<String>> {
+        require(urls.isNotEmpty()) { "Blossom server list must contain at least one server" }
         return urls.map { listOf("server", it) }
     }
 

app/src/test/kotlin/com/wisp/app/nostr/BlossomTest.kt

@@ -128,4 +128,30 @@ class BlossomTest {
             result
         )
     }
+
+    @Test
+    fun `buildServerListTags throws IllegalArgumentException when urls are empty`() {
+        val emptyUrls = emptyList<String>()
+        assertFailsWith<IllegalArgumentException> { Blossom.buildServerListTags(emptyUrls) }
+    }
+
+    @Test
+    fun `buildServerListTags builds server tags preserving order`() {
+        val valid1 = "http://example.com"
+        val valid2 = "https://example.com/"
+        val singleServerTags = Blossom.buildServerListTags(listOf(valid1))
+        assertEquals(
+            listOf(
+                listOf("server", valid1)
+            ), singleServerTags
+        )
+
+        val manyServerTags = Blossom.buildServerListTags(listOf(valid2, valid1))
+        assertEquals(
+            listOf(
+                listOf("server", valid2),
+                listOf("server", valid1)
+            ), manyServerTags
+        )
+    }
 }