Use preconditionFailure for ably-cocoa compatibility guards

Replace fatalError with preconditionFailure and add comments noting
that these are fallbacks which our ably-cocoa dependency version
should ensure are never triggered.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: lawrence-forooghian

Sources/AblyLiveObjects/Internal/DefaultInternalPlugin.swift

@@ -161,8 +161,9 @@ internal final class DefaultInternalPlugin: NSObject, _AblyPluginSupportPrivate.
                 return nil
             }
 
+            // This is a fallback; our ably-cocoa dependency version should ensure that this is never triggered.
             guard (connectionDetails as AnyObject).responds(to: #selector(ConnectionDetailsProtocol.siteCode)) else {
-                fatalError("ably-cocoa's connectionDetails does not implement siteCode. Please update ably-cocoa to a version that supports apply-on-ACK.")
+                preconditionFailure("ably-cocoa's connectionDetails does not implement siteCode. Please update ably-cocoa to a version that supports apply-on-ACK.")
             }
 
             return connectionDetails.siteCode?()
@@ -182,8 +183,9 @@ internal final class DefaultInternalPlugin: NSObject, _AblyPluginSupportPrivate.
         let objectMessageBoxes: [ObjectMessageBox<OutboundObjectMessage>] = objectMessages.map { .init(objectMessage: $0) }
         let internalQueue = pluginAPI.internalQueue(for: client)
 
+        // This is a fallback; our ably-cocoa dependency version should ensure that this is never triggered.
         guard (pluginAPI as AnyObject).responds(to: #selector(PluginAPIProtocol.nosync_sendObject(withObjectMessages:channel:completionWithResult:))) else {
-            fatalError("ably-cocoa does not implement nosync_sendObjectWithObjectMessages:channel:completionWithResult:. Please update ably-cocoa to a version that supports apply-on-ACK.")
+            preconditionFailure("ably-cocoa does not implement nosync_sendObjectWithObjectMessages:channel:completionWithResult:. Please update ably-cocoa to a version that supports apply-on-ACK.")
         }
 
         pluginAPI.nosync_sendObject!(