fix(commons): iOS test compile error and CharsetDecoder interop warnings

Kotlin/Native rejects ',' in function names, so the backticked test name in
UploadFallbackTest broke compileTestKotlinIosSimulatorArm64. Also opt into
BetaInteropApi for NSData/NSString.create and drop the NSString-to-String?
cast the compiler flags as only succeeding on null.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019fqLYHsGZQLj55XxoB6RN7

commons/src/commonTest/kotlin/com/vitorpamplona/amethyst/commons/model/mediaServers/UploadFallbackTest.kt

@@ -34,7 +34,7 @@ class UploadFallbackTest {
     }
 
     @Test
-    fun `a server is never tried twice, even under another spelling`() {
+    fun `a server is never tried twice even under another spelling`() {
         val primalAgain = ServerName("primal", "https://BLOSSOM.PRIMAL.NET")
         assertEquals(listOf(primal, band), blossomUploadOrder(primal, listOf(primalAgain, band, band)))
     }

commons/src/iosMain/kotlin/com/vitorpamplona/amethyst/commons/preview/CharsetDecoder.kt

@@ -20,6 +20,7 @@
  */
 package com.vitorpamplona.amethyst.commons.preview
 
+import kotlinx.cinterop.BetaInteropApi
 import kotlinx.cinterop.ExperimentalForeignApi
 import kotlinx.cinterop.addressOf
 import kotlinx.cinterop.usePinned
@@ -41,7 +42,7 @@ import platform.Foundation.create
  * to their `NSStringEncoding`; anything else falls back to UTF-8 (matching the
  * "defaults to UTF-8" behaviour of the charset sniffer).
  */
-@OptIn(ExperimentalForeignApi::class)
+@OptIn(ExperimentalForeignApi::class, BetaInteropApi::class)
 actual fun decodeBytes(
     bytes: ByteArray,
     charsetName: String?,
@@ -55,7 +56,7 @@ actual fun decodeBytes(
             NSData.create(bytes = pinned.addressOf(0), length = bytes.size.toULong())
         }
 
-    return (NSString.create(data, encoding) as String?)
+    return NSString.create(data = data, encoding = encoding)?.toString()
         ?: bytes.decodeToString()
 }