Make a call with no UI actually run, and cover it with tests

Found on a device: with no UI to answer the microphone permission or unset the maximized snapshot, the call parks forever and blanks the screen through the proximity wake lock.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

features/callnative/api/src/main/kotlin/io/element/android/features/callnative/api/NativeCallEntryPoint.kt

@@ -19,6 +19,6 @@ import io.element.android.features.call.api.CallData
  * There is nothing behind this seam yet - the default implementation does nothing and says so. The
  * native stack that implements it is a separate library, added in a follow-up.
  */
-interface NativeCallEntryPoint {
+fun interface NativeCallEntryPoint {
     fun startCall(callData: CallData)
 }