fix(mac): preserve AltTester assemblies from UnityLinker stripping

Unity 6 macOS builds run UnityLinker managed stripping and the AltTester
runtime scripts were ending up missing at runtime (AltTesterPrefab missing
script), preventing the AltTester server from listening on port 13000.

Add a link.xml to preserve AltTester assemblies.

Author: rodrigo-fournier-immutable

sample-unity6/Assets/link.xml

@@ -0,0 +1,13 @@
+<linker>
+  <!--
+    Preserve AltTester types on Unity 6 macOS.
+    The macOS Unity 6 build uses UnityLinker managed stripping (see CI build-log),
+    and we observed missing-script issues on AltTesterPrefab at runtime which
+    prevents the AltTester server from listening on port 13000.
+  -->
+  <assembly fullname="AltTester.AltTesterUnitySDK" preserve="all" />
+  <assembly fullname="AltTester.AltTesterUnitySDK.Driver" preserve="all" />
+  <assembly fullname="AltTester.AltTesterUnitySDK.Editor" preserve="all" />
+  <assembly fullname="altwebsocket-sharp" preserve="all" />
+</linker>
+