Survive stripped UnityEngine.Application members during chainloader init (#1391)

Two related failures on aggressively stripped Unity Mono builds:

1. InitializeLoggers reads Application.unityVersion, which the managed
   code stripper can remove. The version is only used for an
   informational log line, but a MissingMethodException there aborts the
   rest of chainloader initialisation. Read it defensively instead.

2. UnityChainloader.Initialize caught every exception, logged it as
   Fatal, and returned normally. StaticStart then called Execute() on a
   chainloader that was never initialised, so BepInEx logged
   "Chainloader startup complete" while every plugin silently never ran.
   Rethrow so a failed init cannot be mistaken for a successful one.

Verified on Beacon Pines (Unity 2021.3.45f1, Mono, x64), which strips
both Application.unityVersion and the log-callback members.

Co-authored-by: ManlyMarco <39247311+ManlyMarco@users.noreply.github.com>

Runtimes/Unity/BepInEx.Unity.Mono/Bootstrap/UnityChainloader.cs

@@ -60,6 +60,26 @@ public class UnityChainloader : BaseChainloader<BaseUnityPlugin>
         get => Application.unityVersion;
     }
 
+    /// <summary>
+    ///     Reads the runtime Unity version, returning null if it is unavailable.
+    ///     Unity's managed code stripper can remove Application.unityVersion entirely, in which case
+    ///     touching it throws MissingMethodException. The version is only used for an informational
+    ///     log line, so it must not be able to abort chainloader initialisation.
+    /// </summary>
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static string TryGetUnityVersion()
+    {
+        try
+        {
+            return UnityVersion;
+        }
+        catch (Exception e)
+        {
+            Logger.Log(LogLevel.Debug, $"Could not read the runtime Unity version: {e.Message}");
+            return null;
+        }
+    }
+
     [Obsolete("This method is public due to a limitation with Unity 4.x. DO NOT CALL", true)]
     public static void StaticStart(string gameExePath = null)
     {
@@ -116,6 +136,12 @@ public class UnityChainloader : BaseChainloader<BaseUnityPlugin>
         {
             Logger.Log(LogLevel.Fatal, $"Unable to complete chainloader init: {ex.Message}");
             Logger.Log(LogLevel.Fatal, ex.StackTrace);
+
+            // Rethrow so callers do not go on to Execute() a chainloader that was never
+            // initialised. Swallowing here produces a far worse failure mode than crashing:
+            // the loader carries on to log "Chainloader startup complete" while every plugin
+            // silently never runs, which is extremely hard to diagnose.
+            throw;
         }
     }
 
@@ -125,11 +151,16 @@ public class UnityChainloader : BaseChainloader<BaseUnityPlugin>
 
         Logger.Listeners.Add(new UnityLogListener());
 
-        // Update version info from runtime in case it wasn't set yet
-        var prevVersion = UnityInfo.Version;
-        UnityInfo.SetRuntimeUnityVersion(UnityVersion);
-        if (UnityInfo.Version != prevVersion)
-            Logger.Log(LogLevel.Info, $"UnityPlayer version: {UnityInfo.Version}");
+        // Update version info from runtime in case it wasn't set yet.
+        // May be unavailable on aggressively stripped builds; it is informational only.
+        var runtimeUnityVersion = TryGetUnityVersion();
+        if (runtimeUnityVersion != null)
+        {
+            var prevVersion = UnityInfo.Version;
+            UnityInfo.SetRuntimeUnityVersion(runtimeUnityVersion);
+            if (UnityInfo.Version != prevVersion)
+                Logger.Log(LogLevel.Info, $"UnityPlayer version: {UnityInfo.Version}");
+        }
 
         if (!ConfigDiskWriteUnityLog.Value) DiskLogListener.BlacklistedSources.Add("Unity Log");